Specific muscles or movements patterns and associated muscles can be under recovering, overreached or overtrained.
Wouldn’t say you’re overtrained without any systemic symptoms but sounds like your lower back is under recovering given your current training. Even in well designed programs based on the big three the lower back takes a hammering, big loads, volumes and frequencies. However a good program should still allows you to recover.
Muscle soreness after workouts will decrease over time but chronic under recovering is at best limiting progress and more than likely setting you up for some kind of injury…
There’s lots you can do: Revaluate your split/program, reduce work on the low back as much as possible, increase recovery ability.
Not gonna go through programming here because it’d just be rewriting your program. You can do your own research on programming or get a proven program off the internet that fits you.
There’s loads of considerations to be made here’s some examples:
You’re doing a lot of concentrated volume. 10 sets of squats on one day sounds like it’ll definite make you sore. 5 sets twice a week is the same volume but you’ll recover from it.
Frequency. After training muscle protein synthesis goes up a bit for a few days. You wait a week to train and you’ve missed out on a few days potential gains.
Daily/Weekly/Monthly Volume: Bit different for everyone but there is legitimate point where you are doing too much work to fully recover i.e. Maximum Recoverable Volume, regardless of how much you eat or sleep. If you train with too little volume you miss out on potential gains. You train too close to this limit or above and you slow down or plateau as your body is forever trying to catch up with recovery.
There’s much more to consider when programming. Anyways the point is unless you’re confident enough in your knowledge/experience to put together a workout program then it’s best use existing programs or at least draw from them and use them as templates.
Unless you are incredibly attach to body part splits alternatives such as Upper/Lower, Push/Pull/Legs or Full Body can enable you to train body parts 2 times or 3 times a week successfully.
You seem to have lots of freedom or spare time to dedicate to training make good use of it by choosing a good program.