After Effects alternative - mainly for 2d motion design and animation
If been, looking my self from time to time to find an alternative to after effects, recently got rid of my adobe subscription - but still could not replace that specific piece of software. Maybe someone has some good experience?
Natron is essentially the FOSS version of Nuke. And Nuke may seem overkill, but using it for simple tasks at first is a great way to familiarize yourself with the tool before using it for more complex ones.
I used to use Nuke just to do some colour grading, or composite two animations together, back when I was in school for 3D animation. "Simple" stuff that Blender could've handled, but I liked how Nuke was designed specifically for composition and VFX. The focus helps, I find. Which made me happy that Natron is a thing (although I recall it having some stability issues with me).
Blender is nice but last time I checked it was not well optimised for 2d animation, motion graphics. But maybe it's time to give it another go - I think it's one of the best open source projects ever. The other one you mentioned lack in the motion design department. Gimp still gives me nightmares.
I use it for basic 2D animation - overlays for videos (captions, title sequences, etc.) and animated diagrams - it works really well when you get used to it (mastering the curves editor is essential!). If you're going to composite what you do onto video outside of Blender (I use Resolve) you need to export as an image sequence in a format that supports transparency (e.g. png).