I’ve had knee problems on and off for about 5 years including an ACL repair 3 or 4 years ago now.
My advice is that if you want a stable knee, you need to start from scratch in the gym, preferably with support from a physio. Extended periods of injury lead to all kinds of imbalances and you aren’t going to eliminate them on a bike, in fact it is easy to make them worse.
Training for outright power/strength is very difficult to combine with rehab on an unstable knee. Everything you do needs to be done with a decent posture and range of motion. If you are patient then your leg and core strength will improve and you will find you are significantly stronger and more balanced on the bike.
Weights stuff that works for me.
Light squats, big ROM
Deadlifts
Poliquin/Peterson step-ups
Single-leg deadlifts
Split Squats
Core training is always beneficial, and things like skipping, shuttle runs, step-ups, lunges, squats done in a circuit can be helpful.
It is a very good time to start, if you get into the gym twice a week, make sure you are patient with progression and mix up your exercises you could make a lot of headway before the clocks go forward again.