I think a good place to start would be to not normalize that kind of insanely detailed worldbuilding. ^^;
Like, being able to imagine everything in your fictional universe 'down to the last microscopic detail' is fine; in fact it's pretty cool and impressive, but imagining things and creating a story are two different things. Related, but different.
You don't need that level of detail in your setting. Your readers aren't entitled to it, nor should they be forced to sit through it in lieu of an actual plot. Ideally, you shouldn't even have time for it: you're not writing a textbook, you're writing a story. Any information that doesn't enhance and facilitate the telling of that story is unnecessary.
One thing to take note of is that these aren't the same question. :T Soft Sci-Fi exists, the same way Hard Fantasy does. It all depends on how seriously you take the rules, and how much those rules are emphasized within the story itself.
Now when it comes to combining genres, I think the best thing to do is just take one element you like from each, and force them to coexist. Spaceships and pirates, or knights and cyborgs, or magic and computers...just mash them together, and then worldbuilding will consist of explaining how they can work together. I write a lot of science-fantasy, so I have plenty of experience with this~
For example, magic and computers-- the most basic science-fantasy combo. It's a simple concept with endless potential...which probably tempts many authors to go all the way down the wormhole and use it as an excuse to make a magic system that's as intricate and structured as possible, the way we conceptualize computer science IRL. Which, again, is fine, but there are other options. ^^; For example, instead of forcing magic to play by computing rules, you could try making computers play by magic rules.
Imagine a computer whose workings are literally unknowable. Like, the average person doesn't really get how their smartphone works, yeah...but what if that ignorance was universal?? What if we grew computers out of the ground or hatched them from eggs or had them handed down to us from the gods, and there were entire fields of science dedicated to figuring out how they function and why they exist, to no avail?
Or, imagine a computer that plays the role of a mysterious wizard or fairy in the plot. It's the only object of its kind, hidden in a secret place far from the rest of the world, and you can go up to it and ask it questions that no mortal being can answer. Maybe you have to collect a series of secret treasures in order to get it to work (like its batteries ^^; ), or maybe it can refuse you or trick you if you aren't careful.
Or, or...what if you used magic entities to recreate the functions of computers? This is a pretty common tactic...for instance, in one of my stories, the lingering, semi-conscious souls of the dead are the main power source for society, and their psychic abilities are used to make them into appliances. Souls that can detect people's presence are used for security; souls that can detect energy are used as meters; souls that can read minds are used for long-distance communication, and to create user interfaces for everything else.
I think if you're specifically looking to avoid a complicated setting, ^that's the way to go: take a technological concept and use magic to simplify it. You don't have to go into detail explaining how things work if the ultimate answer is 'because magic said so'.