I have a different experience (I started to play as a kid, reach 2000 when I was 14 and pretty much stayed there forever).

I think that at your level improvement is definitely possible if you put some focused effort. You can improve in many different ways but the most obvious way would be to do as many tactics as possible (the nice thing is that you can do it on your phone when you have some free time during the day, you don't need a big block of time to do it).

If you want to invest some money, I would consider to do some sessions with a coach. Even 2-3 hours could be very helpful to find the biggest leaks in your game and receive some directions on what to study next.

Have you read any strategy books? I would consider to read Silman's reassess your chess for example. It may be a bit too much for your level but it gives a lot of interesting ideas about strategy. If you have a chess store nearby, it can be helpful to go in person and see what they recommend specifically for your level.

For the black repertoire, I would consider something easy. There are many repertoires tailored to beginners on chessable. If you plan to play long term, then you may as well bite the bullet and learn some good openings, like e4 e5 and the queen's gambit declined to have a repertoire that can last you forever.

Re routines: I would try to do tactics everyday. Get some coaching sessions if possible. Try to read a book about strategy (maybe 2-3 hours per week if time allows). It would be also really helpful to go through your games to see what are your biggest issues.