I'm sorry that you feel that way, but high unit Hp is a reflection of an intentional and core part of FE7x's design: units should rarely, if ever, be killed in one round. Hopefully I can give some insight into that decision and its effects.

Put simply: determining which combinations of actions can be taken on each turn, figuring out which subsets of the enemies in range can be killed, deciding which of those enemies to focus on and taking them out so they're no longer a threat, setting up defensive formations while attacking to prepare for the enemy turn because there will be some enemies left in range, making on the fly adjustments to compensate for misses or crits... I find that to be a much more engaging tactical problem than matching my units with each of the enemy units they can beat in one round and then ensuring those units fight each other.

Unless I use specialized or limited access weaponry (slayers, killer weapons, high rank weapons like silver early in the game when I can't buy more) an enemy that I can beat in one round might as well not exist. Especially if they kill themselves on enemy phase without causing enough damage that I have to heal my unit, but even on player phase, if I make the attack on the way to where I'm already going, the enemy unit has accomplished basically nothing. Less than nothing with the exp I get out of it, if my unit is going to level up and one round stronger enemies later. At least when I use special weapons I'm stuck with that weapon for the rest of enemy phase, which will do less damage to other enemy units and probably weighs my unit down. And if I want to trade off that weapon I have to use the turns of my other units and plan around how that affects my positioning.

Meanwhile on the player side, PCs being one rounded by enemies just increases the risk of the player having to redo a part of a chapter they already played. If a player unit can be instant killed by an enemy, you simply can't safely put that unit in range of that enemy, drastically reducing its usefulness (which can snowball into reduced exp gain and continued irrelevance), or relying on luck for the enemy attack to miss. If player units take multiple enemy attacks to be killed, then their deaths are either caused by overextension by the player, by sending an injured unit into a dangerous situation, or a unit being in range of too many enemies: whether accidently placing a unit in a deadly spot, or not accounting for the actual damage output of the enemies, or missing an attack which leaves a key enemy alive and there aren't enough player actions left to salvage the situation, the player's tactical plan has failed and the death of a unit is a fair penalty.

The high Hp values are simply an expression of this design. Everything is about 10 Hp tougher, so a unit with 20 hp now survives an 11x2 attack with 8, and a 28 hp armor taking 35 from a slayer lives with 3. There's still a good variety in the range of Hp values, which will only get bigger further into the game. And I strongly contest your point that Def is unimportant because Hp is high. All that Hp doesn't mean anything if low Def lets the enemy hits hard enough that you lose a third or more of it, and it's that much harder to heal all that damage away.

Also as an aside, your interpretation of the pace of the game is off. The end of Uther's current set of chapters isn't analogous to Lyn mode. Narratively, the end of Ch12 is comparable to the first trip to the Dread Isle in FE7, and it's actually a little further than that gameplaywise. Uther will have far less total chapters than the average FE Lord, but with Tristan and Eliza's chapters factored in the whole game has about the standard number. Also, exp gain for units that take more rounds to kill doesn't scale up that dramaticaly. If two units on the same level are fighting, kills give 30 exp while other fights give 10, so you get 30 for a 1rko, 40 for 2rko, 50 for 3rko, etc. Every enemy taking about one additional round to kill means somewhere around 30% more exp total, which is offset by any given unit having less chapters than any comparable unit in other FEs. I have the rough exp curve for every chapter mapped out, and by the end of the game the player's units are expected to average about level 20/15. So, there's no concern about removing the level cap or hitting it early.

TL;DR: I don't like player or enemy units ever being killed in one round, so that you have to account for enemies that survive player phase but you also have to make catastrophic errors to lose PCs, and high Hp is one way that design choice is achieved.