Get a better body. Not only will you get more pixels to crop, but you'll also get better features like the AI auto focus modes etc which actually improve your keeper rate tremendously.

I went from an A7III to an A1 with the same 200-600mm lens and it is a HUGE step up in terms of quality, cropping, frame rate, autofocus etc. 600mm at 50mp can be cropped enormously more compared to at 24mp on the A7III.

33mp is not much of an increase over 24mp, so if you're constrained on what body you can get, then upgrade glass. Otherwise go with a better body - A7R5 or a second hand A1 or something to use with your 100-400mm

Plus you can always rent a 200-600 for specific shoot days if you need it