I know that a bike is supposed to be high enough that your leg is almost extended at the bottom of the movement. But is that mainly to be able to exert maximal force? Or is it also important for not fucking up your joints / muscle balance / etc?

After some theft and vandalism issues I'm considering switching to the local rental bike system for my commutes, but I'm a pretty big guy (195 cm) so the bikes aren't big enough for me. Even when the saddle is raised to max height, I reckon it's still at least 10 cm too short to reach the "ideal" position.
I just did a 20K trial ride, and my knees felt kinda "wrong". Not real pain, so I reckon it's theoretically possible that I just need to get used to riding like this and eventually it'll feel normal etc. I also noticed that I was even slower than usual (15ish KPH instead of 18ish), but if that's the only downside I don't really mind.
But I don't want to fuck anything up. And if continuing like this is a bad idea, I would just like to find an alternate solution straight away... (which would probably entail buying a second-hand bike and arranging a safer parking spot in an apartment building nearby).
Some extra info if it matters : I don't have a lot of biking experience (only learned to ride this year). I'm 33. Regular commute is 30K one-way. I traditionally used my bike about one day per week, but I would ideally like to increase that.

Thanks!