How to fix bad knee mobility

I started Jiu-Jitsu about 8 months ago and something that I have consistently noticed is how my bad knee mobility impacts the performance of the most basic techniques. I cannot shoot doubles and singles with explosion since "shooting" requires knees-over-toes mobility, and I cannot quickly transition to armbars from mount for a similar reason. When it is required to get up off your knees quickly for passing, I also find that challenging. I started doing split squats this week; also, I am 6'1" and 250 lbs, so maybe weight loss could be a solution. What do you guys think?