HAL is great for the start, I mean, coming from much more simpler Pic/Avr, you'll get completely blown away by the endless registers, bits...
After some weeks/months doing stuff, analyzing the generated code, the HAL libraries, all the gears in your brain will slowly start spinning again.
Then switching to LL is much easier.
Still takes a considerable amount of work if you have a lot of code interfacing the hardware, but gets done eventually.
The savings vs time invested depends on your application, if you had to take a more expensive stm32 due running out of space and you're buying 1000s of them, might well deserve the effort, specially nowadays when the same thing with extra 128K flash might cost easily 2x, 3x more because it's hard to source.
Otherwise, it's not a miracle either, removing the HAL handlers and the bloat might save 20-30%, but it's not LZMA
I've been practicing LL by porting the soldering firmware, not yet done, but getting there.
As always with ST, there's a lot you must figure yourself!
Honestly, did zero RTFM, might have a look but I don't expect anything useful there!