how long do you plan on making mirror monster?
the thing with long series is the longer they get, the harder it is to maintain and develop growth momentum if you havent already got the ball rolling. your comic is doing pretty well for its age actually and you should be proud of yourself, but getting from 150 to 1k at this point is unlikely. thats fine, too - niche is good, popularity doesnt necessarily mean greater merit
however, because youre so young and will be developing rapidly in the coming years, id worry about working on this for more than 3 years becoming really unrewarding (in terms of career and emotionally). so if your plan for the future of mirror monster is vague, maybe start looking at wrapping things up in a year and a half. i highly recommend finishing this - you clearly care for the story, which is a pretty good story, and finishing a project is a really important part of your journey as a creator. but once this is done, youll have it all under your belt, and can move onto your next project with a great deal more foresight and experience.
re: building an audience now, though - yes, get back on twitter. im shit at it myself but its a really wide reach with some great hashtags to use. but since you already use it, id suggest focusing on instagram. your insta is good - youve got a decent base of followers and the work you post is quality, but it could be a lot more engaging. why not post some sketches, wips, or experiments? some videos of you drawing would be good to shake it up.
also, hashtags. hashtags save your life. i recommend adding #webcomic #tapastic #digitalart #clipstudiopaint (if thats what you use) #comicsoninstagram #indiecomics #supportindiecomics #lowbrowart #theillustrationroom #drawingoftheday #webcomicwednesday #webcomicchat #indiecomicsart to your roster. i find that supportindiecomics and lowbrowart get me the most traffic of this lot
post update notifications!! you post page screencaps which is really great, but have you considered making update graphics? eg:
(niahs is way better than mine, i need a revamp
)
instagram is a visual medium (duh), so if you put all your key info in your image and make it striking, you dont even need to worry about the caption!!
also, posting regularly (once a day is my routine, dont go for more than that id say) on instagram really helps - i use later.com to queue up posts weeks ahead so i dont need to worry about it too much, and it really helps.
aaaaaaaaaaaalso follow more people!! engage with them! follow hashtags to find new people. developing a community is how you get people to care about your work, and you can also find new friends and allies in the comic biz.
these are only a few small tips, im still figuring out promotion and comms myself, but i hope they help! to repeat, you have a really good first comic on you and while reaching high popularity is unlikely, you can and will foster a positive community around your work, and the artistic development alone is worth it. dont compare yourself to others, theyre playing entirely different games, with entirely different goals.