Getting better

I usually only pubs with friends so I can coordinate ganks/lanes/picks with them. I only have a few games solo. I want to be a better player in general with flexibility in terms of roles. I always pick depending on my team composition. Would playing ranked solo slowly make me a better player? People always say solo ranked at my tier is hell. Is it even possible to get better MMR since you need to have a positive winrate to achieve the aforementioned higher MMR?

Solo and party MMR have their own advantages in terms of learning, though parties are usually better

If you do want to get better at the game, start watching some replays and pinpoint what problems you have, and work on them. If you want to make yourself flexible in terms of roles, start practicing them with your friends, and see what problems you have with each role. I personally have started practicing more farm-oriented heroes (I usually play nukers like Puck) and a major problem I have is that I have poor farm efficiency (I will get only around 70-100 last hits when we're 20 minutes into the game already) so I'm practicing last hitting and such.

Would ranked solo make your a better player?

It probably will, but party practice will be much faster, so I would stick to parties for now. I play Solo when I want to raise my Solo MMR, but I do prefer playing with friends more (I'm not too bothered by my MMR, I just do it more as a friendly goal I have).

You can get better MMR, but you have to be way better than your current MMR level to do so if you're trying to raise Solo MMR. Solo MMR is heavily skewed by your teammates that you have little communication with, so if you have even one bad teammate you could end up losing that game. Party MMR is more accurate since you can control what your team does, or at least know exactly what the team is doing.

MMR is calculated through a lot of things

Having a positive win rate doesn't ensure a good MMR, MMR is calculated through a lot of things. Ranked solo won't make you a better player, to be a good player you have to be able to have a good game sense, good direction, coordination and collaboration. You can easily become a good player without playing ranked solo, what you can learn from ranked solo is how to play with strangers and trusting them. I've done my ranked solo placements and haven't gone back since, I was happy with what I got and now play ranked party from time to time.

You can learn to trust strangers by not playing in a 5 stack