I haven't played Warframe at all in a few years. What's a selling point for someone who got a little tired of the grind, but enjoyed the core gameplay?
If you enjoyed the gameplay, it's still exactly the same - buttery smooth! It's the main thing that keeps me playing this game as well (that and years of attachment). I would give it a try again just for the new story bits you must have missed. As for the grind, my best way to not burn out it to take is slow and only grind for things I actively want/need. Take Gauss here, don't care about the frame, happy to just passively get some of his pieces while cracking relics for Forma BPs and maybe trade for the rest if I feel like it in a year or two. Also, take breaks when the content gets boring, Warframe is nice enough to have almost no FOMO, even Nightwave rewards keep coming back regularly.
Aya/Prime Resurgence was a pretty big upgrade over the prime vault system, and even Nightwave became less grindy this season with the elite weekly challenges that essentially complete themselves making it easier to passively get the cosmetics.
I generally like making meme builds, and playing around with gimmick weapons, even though they're generally outclassed.