I've been playing since release. It's better than it used to be. The degree to which it is better is debatable, but all aspects (diplomacy, tactics, empire management) have improved noticeably.
Can they compete to a certain degree?
Yes. King provides a decent challenge if you are new to VI but not to Civ. Emperor after you've learned the ropes and are working towards playing optimally. However, you can eventually outgrow Emperor and then you're into Immortal and Deity, both of which give the AI massive bonuses that aren't necessarily fun to overcome. You spend a large chunk of the game on the back foot before you finally catch up and surpass the AI. Once you've done that, though, they stand no hope of catching up. The fun at that point is in learning to play progressively more optimally and in achieving victory in fewer and fewer turns.
I agree with the discussions surrounding the AI's continued tactical weakness. I'm not certain removing 1UPT is the magic bullet required to fix the issue. I think doomstacks mainly allow the AI to leverage it's production bonuses on harder difficulties to mindlessly throw waves of units at you and hope to win by attrition. That may be more challenging than watching the AI struggle to shuffle its units around, but I don't think you can call that sophisticated AI tactical execution.

Having said that, I'm not opposed to the return of stacks; I was OK with them then and I'd be OK with them returning if it would actually help the AI compete militarily. Or just for the fun of it, stacks were entertaining in their own way. I have to admit that the tactical challenge of unit movement and positioning is fun to me as well, it was definitely worth trying in V and VI.
Honestly, I find the AI's military management far less disappointing than the AI's empire management. Cities in strange places, cities with districts in terrible places (when better options are clearly available), cities with resources unimproved far past when they should have been, cities in the line of fire without walls even after war has been ongoing for many turns, the list goes on and on. Giving the AI ludicrous bonuses on harder difficulties really doesn't fix this problem, it just papers over their inability to compete throughout the course of a game. It also makes capturing AI cities less rewarding than it should be, since they have a decent chance of being painfully sub-optimal. Even if Firaxis would just take the time to have the AI optimally place districts it would help immensely. Look how well Korea does in regards to science, just because the AI is forced to place +4 adjacency Campuses. Now imagine every AI is able to assess the three tile radius of its cities, note the optimal district locations, buy tiles to place them optimally if required, prioritize districts that will get particularly good bonuses (weighted by that Civ's victory biases), clump dams / aqueducts and wonders to create adjacency options for IZ's and TS's respectively, etc. It would be a real game-changer and would help to keep the AI competitive into the modern era, injecting some fun back into the late game.
I'm not sure if the things preventing this are coding resources or computing resources (or both). Either could be fixed (hire more / better AI devs, offload AI to the cloud) but so far we've just seen improvements around the edges. Good, worthwhile improvements, but they don't address the heart of the problem. After four years, I think this is (mostly) the AI we're going to have going forward. But who knows what Firaxis has up their sleeve.