Thanks, it's still a work in progress
For me personally the topspin FH is actually a harder shot to master than the slice BH or kick serve, but I grew up playing other sports such as baseball, football, and basketball so that could be a factor.
My tennis coach taught me the BH slice when I started playing tennis around 12 years old. He was old-school and made me learn the BH slice before the BH topspin! He previously played D1 college tennis and had one of the best slices I have ever played against.
The main checkpoints for a high-quality offensive slice, IME, are: 1.) racket takeback behind the head, 2.) stay sideways through the shot, 3.) weight transfer onto front foot when possible, and 4.) racket follow-through to the side with offhand extended behind you for counterbalance. Once you get the proper form down, then hit about 10,000 reps with hand feeds and/or ball machine and you'll be good to go for live ball
IMHO, if you're trying to improve your slice, I think self-feeds are a good starting point to work on the proper form. Try to master self-feeds before moving to live ball. Best of luck!