The "1999" expansion coming up seemed promising, 
I've always known Warframe is like an asteroid belt of content updates where each asteroid floats somewhat independently of one another but trying to put it together is actually discouraging me from diving in.
I see stuff that sounds very cool, like some new fancy way of upgrading my old Bo Prime to be a stat stick of increased sprint and bullet jump, but then its like "oh you have to go to yet another grindy hub and collect a lot of stuff and probably do daily leveling tasks with local rep" which was what eventually made me set the game aside; during The New War that part with the mandatory Necramech sequence was such a story-momentum-stopping bottleneck (because I didn't have a Necramech and I didn't do the grinds necessary to get one because I hated that particularly goopy and unpleasant hub area) that I paid some real money, got a quick and dirty Necramech unlock, and only then completed it, felt tired... and logged out for years.
Also, as silly as it may sound, the notion of "feeding" my old Warframes to the Helminth to min-max my action bar felt so whiplash-like counter-intuitive to everything my Operator's story had built up to at that point that the decision to stop logging in was partially pushed by that once that was the new hotness content.
I remembered this resonating with me:
"...We had created monsters we couldn't control. We drugged them, tortured them, eviscerated them... We brutalized their minds... but it did not work, until they came... And it was not their force of will, Not their void devilry, not their alien darkness... It was something else. It was that somehow, from within the derelict-horror, they had learned a way to see inside an ugly, broken thing... And take away its pain.''
And now it's "ah naw, go get fed into the talking tumor, I want a better action bar." 
I guess my question is this: where does an old veteran even start with the avalanche of new content? Is it all rep grinds from content asteroid to content asteroid from here on? As silly as it may be to ask: is it still fun?
The Lore's always been kinda murky, but I understand the original frames were made by infecting people with the helminth strain of the technocyte virus (e.g excal umbra, some of the primes), which are what the tenno connected with, but then they were used as blueprints to mass produce frames without the person. The ones you build in your foundry are copies, so you're just returning a lump of infestation to the original mass for it to reabsorb and learn from.
There haven't been many major updates since the new war, so you're not actually all that far behind. Lots of hot fixes, QoL, and updates to bring the content islands closer together. Nothing earth shattering, but some improvements.
Seeing as you're coming back, might as well start with the veilbreaker content that follows on from the new war - archon hunts are the new sorties and Kahl's missions are mini new war sections with various challenges, both weeklies. Zariman kinda sucks as a hub, but the missions are trying something new. Albrecht's lab is similar, but better executed, and has an extreme difficulty weekly if you're into that kind of challenge. Duviri offers a new version of open world without dailies, but the cool stuff from steel path does require a daily rep grind from the Zariman.
Even the foundry-manufactured Umbra my operator made seemed quite capable of suffering and indeed had pain that was in need of being taken away. I can't help but assume that applies to the other Warframes as well, so it's still kind of offputting for me to consign them to the talking tumor for a better action bar, even if there's a meta-level of "it's just fiction" involved. I hope that doesn't nerf my play too much.
the other frames are copies. Umbra is actually the only true warframe you manufacture and use. remember his foundry recipe has kuva. that nears its his original consciousness.
Even the manufactured Warframes seem to have some capacity to think, feel, and suffer. When the Operator's true self was exposed (and almost killed by the Stalker), the Warframe got up unprompted and fought back and carried the kid off.
I always thought that was us remotely controlling it. I mean we did earlier in the quest when we had to get out of the reservoir, right?
A lot of the emotional weight of the scene shifts and becomes almost silly if that is the case. Maybe that was the case, but it retroactively makes that moment less touching for me if so.
yeah....hmmmm
I haven't interacted with the Helminth system at all and I still can survive into roughly level 110 enemies mostly because I'm rusty af and nowhere near the level of mechanical input I was at back during the New War; I really don't think it's that necessary-- ad as it is, I don't like breaking up the action theming of my frames in the first place; like how tf my Frost look popping Volt techniques or smth?
Idk, I haven't noticed any real deficits from not interacting with it yet.
It's completely unnecessary for all content, but I'm not ashamed to use it for goofy shit like Aquablades Grendel (slicey meatball) or Wrathful Advance Nyx (lets you teleport around while using her ult). In the end I'm here to play a fun game, and the buildcraft opportunities are too good for me to pass up.
That said, it does offer a handful of abilities even if you never feed it warframes (-75% ammo consumption is real fun to fuck around with), and weekly stat boosts to random frames. I think you also need it to slot archon shards, which are little stat boosts you can get post-New-War. Treat yourself to a little extra parkour speed and energy capacity without needing to spend a whole mod slot on it.
There's a handful of abilities that can turn a specific build up to 11, but it's as much a way to replace an ability you don't like with something you will use as anything else. I don't think I have any in my level 300+ builds - the one I use most is the ability that autosolves the next lock you encounter to speed up spy missions even more, and that comes from the helminth itself instead of a frame.