I never got that into Civ, so maybe it is mostly my ignorance of 'proper' play speaking, but I actually rather liked the change to non-stacking units as it seemed to make the combat more tactical and less an exercise in simply who had the bigger numbers.
The two big problems with it were that a) the computer player coding was just shockingly bad at dealing with it, particularly around any sort of geographic chokepoints, and b) it was incredibly tedious to deal with moving more than a handful of units.
Many other games like Total War series or Age of Wonders dealt with the overall question by having split strategic and tactical game modes, and they're hardly alone in that. Civ5 and 6 tried to keep the bastard child of those two game concepts in the same strategic-map and it just doesn't work that well.
There are a lot of subtle consequences of 1UPT as well. You can't let the player produce too many units, or the whole map will lock up. You can't make units too expensive though, or they'll just be hitting next-turn 40 times between each production in the ancient era. So tiny cities have to punch above their weight, and big ones below. Once you do that though you have to heavily penalize expansion. What they really needed was to increase the tile-density of the map several times, give room to maneuver. That would seriously change the city management gameplay though, potentially make it too tedious.
I can see a split of using large hexes for cities and strategic deployment but having 7 child hexes inside each strategic one for precise unit movement working out. Effectively limiting stacks to 7 units for combat, and also potentially letting a few groups of units move around each other with multiple turns of micromanagement.
The corp/army mechanic could also be tied into that.
I disliked civ6 due to the warmonger penalty. It seems like all the other civs will dislike you for hundreds of years for waging even a defensive war agressively.
That is fixed with the Gathering Storm expansion. Warmonger Penalties are replaced with Grievances, which each civ tracks individually against each other civ. So long as you have more Grievances against a given civ than they do against you, other civs do not care what you do. It makes fighting defensive/retributive wars much easier. I think it works pretty well overall.
That's why you just make sure you get 3-5 artillery units able to move into position and attack a city on the same turn, and have shielding melee units. Don't give the NPC civ's time to fight back.
You can still protect your artillery, although it's more complicated. You may need to position melee units tactically so that they and their zones of control [0] prevent enemy units from easily (and quickly, during a single turn) reaching your artillery units. Using terrain tactically not just to get a defensive edge in combat strength but also to make your ranged units less easily reachable to enemy melee units may also become even more important.
What I find most difficult about protecting your artillery in Civ 5 is attempting to siege cities with units that have a range of 2, because they'll be vulnerable to bombardment from the city no matter what.
But yeah, the artillery is more vulnerable in Civ 5. You can counteract some of that.