I am pretty skilled knowledge wise at speedrunning and have been doing it for a while and ive been practicing FSG, but each time i get a super good seed, one that could probably sub-10, I end up getting really bad times because I spend too long inventory managing and crafting. How can I improve at those?

Practice. And for inventory managament try sorting the most important stuff while doing stuff like killing blazes or waiding for trades

It’s entirely just practice, over and over again. Also, if you aren’t already, do everything with hotkeys, it’s much faster.

I also spent too much time managing my inventory. I stopped trying to organize my inventory as much and started only focusing on organizing my hotbar. The only 2 times I really mess with my inventory is when I'm doing gold trades or when I need room in my inventory to create beds for the ender dragon. But like the other people said, it's just doing it over and over again.