I guess with the left hand you should practice in slow and fast tempo both.
I’m not very good with my left hand too but I have my own story of improving.
Since my left hand was injured it was hard for me to use it. For example I couldn’t do pulloffs with pinky or ring finger at all. I still have some problems but it’s became much better recently.
What I did was developing additional strength. I used a gate with a very fast attack, then I turned threshold until I could barely hear my usual hammer-ons and pull-offs. Then I played scales and licks using left hand only, trying to make them audible. This required to have strong sharp hammer-ons and a bit forceful pull-ofs . Then I turned threshold up a little and repeated the whole process (which made me to do even more strong ho/po).
After some time of this procedure I gave my hand some rest, then turned the gate off, cranked the amp, and played the same stuff trying to use just as little force as possible to get acceptable sound.
After a week of this ‘practice’ (which I did more for fun than for practicing purposes) my left hand became better.
P.S. It worked for me but it wouldn’t necessary work for you since we have a bit different problems.
P.P.S. If you still want to try this method - don’t overdo it. I mean I have playing piano since I was a kid so my fingers had some ‘training’ ) But I wouldn’t recommend to do forceful part of this exersise for more than 10 minutes a day.