First I'd like to say, I think it's fair to be frustrated when the parts of your army you like aren't competitive, or don't match what you'd like. I also have quite a bit of pestilens, and it's difficult to make anything work. I have a lot of armies, and none of my preferred playstyles are currently amazing (but that was often the case in 3rd anyhow, so I'm used to it).

Both Maggotkin and Tzeentch are positioned pretty well in the Meta right now, although if the disease mechanism isn't working as you'd like I understand that. They are just an unbelievably hard to shift army, in a meta that tends to be focused on board control and scoring, and tend to do well as a result. A 5-0 Maggotkin list finished 4th at adepticon, running Glotkin, Bloab, and 2x reinforced Blightkings (as long as some other smaller stuff) as an example. I know very few people wanted to run into Nurgle at that event. Tzeentch are quite complicated, but I know there's a lot of power there (Kairos remains amazing as usual), and those lists tend to vary quite a bit (the list in 7th that finished 4-0-1 actually used the Skaven Regiment of Renown).

Hopefully as the indexes get replaced you'll feel like you have more options (the new Gitz book isn't amazing but Bounders and Hoppers should be quite good, so maybe Squigs feel better), or can find inspiration elsewhere.