You need a periodization scheme that focuses on growing some subset of muscle groups at any given time.
The trouble you are going to run into is that it’s really hard to get enough volume on every muscle group when you’re only lifting twice a week, unless you’re spending an unreasonable amount of time in the gym. You’ve got six working sets of squats per week. That’s just not enough volume to grow, especially at your training age (you mentioned 5 years in a comment).
However, maintaining muscle mass takes even less volume than you are doing now. What this means is that you can periodize your training to focus on getting enough high volume to grow one or two muscle groups, while doing just enough on the rest to maintain, switching it up every 8-12 weeks.
So the way I would set this up is to have three mesocycles lasting 8-12 weeks each: a squat, a push, and a pull. On your squat cycle, most of your volume would be dedicated to your squat pattern and knee extension movements. You might do something like 6-8 sets of 6-8 reps of challenging back squats each day, then do some leg extensions or leg press for additional, lower fatigue volume on the quads. Then, with the remaining time on the two days, split up the rest of the movements to get just enough volume to maintain.
For a pull cycle, I’d do something like conventional deadlifts from the floor one day, then RDLs the other, and do barbell rows and pull-ups both days, getting in 6+ working sets on your pull movements, and finally, as before, use the rest of the time to get maintenance volume in for pushing and squatting.
For the push cycle, I would just barbell overhead press both days, barbell bench one day, and dumbbell bench the other for 6+ working sets on all movements. And as before, maintenance volume with the remaining time.
A periodization scheme like this allows you to get enough volume to grow on some muscle groups, some of the time, and over time you hit all the muscle groups. I don’t see any other way to get enough volume in without adding a third day or spending 3-4+ hours in the gym each day (which comes with its own fatigue management complications). It will take longer to gain mass, but that’s the price you pay for the conditions you’re working with.