I'm not going to tell you how to prioritize your practices, but zeroing out FTs would be a very unique strategy.
I generally keep 8-12 pts on a player until they go orange, when I may scale back to 7, if they have other important areas to work on. Under 7, and a player may start dropping (I've seen some players drop at 7, but rarely). Are FTs important? Well yeah, especially if you are talking about a guy who is going to try to score inside a lot, or if you play a lot of uptempo, or the fastbreak set. If you run triangle or flex, have a mostly perimeter offense, and recruit a lot of guys with green in their cores, then I can see skimping a little on FTs, or even punting altogether, especially for players already red. But as a general team philosophy, I can't see that being beneficial in the long run.
FTs do improve, especially for players with high potential.
https://www.whatifsports.com/hd/PlayerProfile/Ratings.aspx?tid=2542&pid=3551828 Douthitt is a good example, he started with a green D. I knew he would not be a prime scoring option early in his career, but eventually FT shooting will help make him a weapon. At 12 minutes, he's gone from green D to blue C in 2 seasons.