I really think you don't understand why many of us in this thread consider backstab and highground as a problem. It's not about the %to hit at all. It's only about advantages for free (+ disadvantage for highground which means it a godmode)
Rather than being a choice among others, backstab and highground are the only efficient mechanics to have advantages. Everything else (faery fire, the help action of D&D, barbarian's reckless attack, true strike, invisibility, ...) is a bad choice because it cost something.
Backstab and highground doesn't cost anything and many other choices we could have are useless/suboptimal because of this... Players always have to use the same "tactics" over and over again whatever their classes or their party build.
The lack of other good choices is the problem and this is only because in combats, backstab/highground advantages aren't balanced compared to everything else.
They probably choose easy advantage because "missing is boring". And that's why some of us suggest new bonuses that doesn't exist in D&D in addition (like bless, +1 weapons, the missing +1 arrows,...)
You consider an AC of 19 but this is only what we can have... Not the majority of our ennemies. I just checked in the game.
Minotaurs = 14 Bulette = 17 Gnolls = 14 Goblins = 12 - 14 The Hag = 15
Let's do it with an AC of 15 even if the AC is just a value they can easily increase / decrease.
Melee +4 modifier +2 proficiency +1 weapons = 65 % without advantages - 88% with advantages
Ranged +4 modifier +2 proficiency +1 weapons +2 highground = 75% without advantages - 94% with advantages
Caster +4 Modifier +2 proficiency +2 highground = 70% without advantages - 91% with advantages
This would give you a reasonable %to hit without advantages and a very good one with advantages.
What would it mean for normal difficulty levels ? That you don't always need advantages in normal difficulty levels even if it's still a very good bonus to grab. Highround would compensate a bit , and there's still bless for harder ennemies.
For very hard bosses, players would have to learn/find the tons of possibilities to have it (hide, faery fire, true strike, reckless attack... and things could be added to increase the synergies between our characters like flanking, RAW help,...)
It's up to Larian to explain what advantages/disadvantages are (the first time you hide, the first time you can flank, the first time you cast a spell,...)
What would it mean for higher difficulty levels with ennemies having an increased AC ? That you HAVE to learn how to have advantages. Learning how to increase/control your %to hit would be the key before trying a harder level of difficulty.
It means learning how to wisely use your class skills, the spells, the actions (hide, help, flanking,...)
This would make the game far more interresting and deep. Not more difficult. Just more deep, with more choices, more creativity.
Now you just have to learn Larian's homebrewed to win, starting with highground and backstab... and that's not very interresting to do the same thing over and over again neither in a tactical TB game, neither in a D&D games that is supposed to offer us tons of possibilities (and there are many in D&D to have advantage).
PS : of course I tried with a +2 for highground and an AC of 15 but a +1 could also be interresting depending the AC-range of ennemies in this normal game mode. This is just an exemple.