Well this is convenient, I was in the middle of reading Royal anyway and you're the first person on the thread.
I understand you're currently on hiatus (and I'm really sorry for the circumstances. My thoughts go out to you and your family), so it's a good time to take a break from the comic grind and spend a bit of time studying, planning and thinking about your comic, so you can come back with fresh eyes and some new tricks.
So the first thing to say about Royal overall is that a lot of what I'm going to bring up is stuff that you have clearly been taking steps to improve on since the comic started. You're on the right track with things, so a lot of the advice should feel expected.
Starting out with an easy fix: Spelling. There are a lot of misspelled words in this comic, like "summoning", "volcano", "sandals" etc. If you're lettering in Photoshop, you can actually fix this incredibly easily. Go to edit > Check Spelling. Otherwise, maybe type out your dialogue into a google doc, then copy-paste to your bubbles. It's only a little extra effort for the big boost to how professional the comic will feel.
Some neater speech bubbles would also add some professional polish. Personally I like to use the shape tool in Photoshop to make nice flattened oval shapes and drop them, along with tails, into a shared layer folder with the folder's blending set to a stroke. It's flexible, quick and looks nice and neat.
Moving on to the Art...
One of the things that stood out to me, particularly early on during that period where the comic was in black and white, was that it was a bit of a struggle telling the characters apart and remembering who was who. The cast has a lot of slim pale skinned guys with medium-short hair dressed in a t-shirt or shirt and full-length slim fit trousers.
That doesn't necessarily have to be an insurmountable problem, since like... yeah that describes the cast of a lot of shoujo manga like Ouran High School Host Club. The art style has a nineties shoujo look that feels influenced by comics like Saiyuki and Gravitation, so looking closely at how mangaka you're influenced by differentiate characters could be a good place to start. Try to think about different physical builds, posture, face shapes. Even when the cast is all fairly young and trim, there can be a fair bit of variety in terms of bone structure, muscle mass and definition, where the muscles are, bodyfat amount and placement. This is a good example.
Clothing fit and material also makes a big difference. You can get across a lot with how loose or tight a t-shirt is, and whether it's tucked in or not, and similarly with trousers, there's a distinct difference in silhouette and impression between dress pants, khakis, jeans, chinos, drainpipes, combats etc. Obviously your aim should be for all of them to still feel like they're in your style, but see what you can do about expanding the variety of the cast and how you show the differences between existing characters.
Throwing in some more female characters and variety to the skintones when new characters turn up would be a really straightforward way to make the cast feel a bit more varied visually, but I think right now there's a bit of a problem with having a lot of characters, so adding more may not be ideal until some existing plot threads are resolved, so concentrating on refining designs already in the comic might be best.
The other thing in terms of the art is... you probably knew this was coming and have been dreading it, it's backgrounds. I can see a distinct increase in the quality and detail of the backgrounds as the comic goes on, but a little more work would really help add a professional feel to the comic. Don't be afraid to use reference. For towns and cities, google streetview can be remarkably effective for this stuff. You can go to the kind of place you want to reference in google maps, plonk yourself on a street and BOOM: reference, get! When drawing indoor environments, look at photo reference or observe from life for details that really help sell your scenes, like skirting boards, light fixtures, door frames and door handles, shelves and what's on them, furniture and flooring materials. Adding a pinboard full of posters and notes to a school hallway and some scuff marks on the floor can add loads of character with relatively little effort.
On the scenes where there's just a panel with a character, consider using some light patterns, texture brushes or noise. Sometimes the soft gradients feel a bit empty and also a little early 00s looking.
Story thoughts:
At times, I felt like there wasn't a strong impression of the overarching narrative, or even who the main character is and what their arc is supposed to be. The comic is named Royal, but at times I really felt like Oliver was the real main character. He seems to get just as much, if not more screentime early on, he has more dedicated and threatening antagonists, romantic tension with hot vampire guy and the battles he's been in felt like they had higher stakes. This does seem to have improved later, with Royal getting a couple of strong action sequences and a mystery set up with this "Lane" person. I think trying to tie the plot threads together and setting up what Royal's main motivation is will help avoid the kind of problems that plagued "Bleach" where every time the mangaka got stuck for story, he introduced new characters to try to put off the problem of a protagonist with no particular motivation or overarching goal he could achieve.
There's a bit of an issue with scenes constantly being interrupted by other scenes that makes things feel disjointed and keeps killing tension buildup. If the scene you're cutting away to has nothing to do with the scene that was cut away from, it should be left til the end of the chapter like one of those Marvel after credits scenes. During an action scene, cut away only if what the other characters are doing is going to impact that battle, so something they're doing or saying explains or foreshadows information about the battle that ramps up the tension (ie. revealing something the character fighting doesn't know, like their opponent has a poisoned knife in their boot! Or their opponent is actually their dad! Or showing a sniper overlooking the battle from a rooftop as she raises her rifle and puts the hero in her crosshairs...).
I'd also advise trying to keep the number of characters in scenes, and just in storyarcs generally, smaller. For one thing, it's easier to draw when there are fewer people in a scene, but it's also easier to follow on the reader's side. If a narrative role can be fulfilled by a character who already exists, and even better, who is already in the scene, so long as it's reasonable for them to be there and they could reasonably have the knowledge and skills to say or do something, have them say or do it. Sometimes a new character turns up and it bogs the plot right down with things that other characters could probably have done. ie. The principal introduces herself over the phone just to tell Royal about the challenge, then she's never seen again. We've met a bunch of other characters, including teachers who probably could have done this and saved time spent introducing somebody.
Overall, it's a comic that's clearly been improving over time, especially in terms of visual storytelling. Hopefully these pointers help give you some direction for continued improvement.