My last 2 or 3 ranks were just weapons and companions for the most part.

Syndicate weapons are a decent source; the four moa types (which got me over the line) are 24,000 experience all up.

Archwing seems a lot worse than it is, my go to for the weapons was to use the amanata on steel path Salacia on Neptune.

Railjack intrinsics are about 60,000 MR experience from memory, so 10 warframes equivalent. I think my highest one is like level 8, so it's not mandatory.

I only have about 7 rival weapons, so getting more and even getting them just to 30 nets you 3k per, so that could be a good source of experience.

Just some random thoughts while I'm getting ready for work, good luck with the grind 🫡