I reread your post a couple of times and couldn't find an actual example outside of Deflect Arrows that seemed to favor the Monk, so I'll respond to what I think your points were. You'll have to be more precise if I misunderstood your points.
TL;DR : Other martial characters will do more for less investment at earlier levels than Monks.
Solasta is actually a perfect storm proving how great Monks are because.
[... smart play...]
Any character will play better if played smart. I don't see how this helps the Monk in its comparison to other martial characters.
Only Monk actually can fare well thanks to the combination of having ranged attacks for free (shortbow proficiency), Dodge as bonus action + Deflect Arrow to cover open ground with minimum risk (in a game with friends the level 3 Monk ended the day with "effective total HP" not far from 90 because could use its reaction nearly one in three rounds to avoid arrows and the Dodge made 9 attacks of 20 miss, including two criticals).
Dodge bonus action on a resource that is shared with everything else. On an AC value that is more often than not below where it needs to be. Deflect Missile is okay, but I suspect you'd need it less with a less awful class.
Your "20 AC is very very bad" is hilarious by the way.
20 AC at max Dexterity + Wisdom. Which usually doesn't happen. Even if you have 16 in both at lvl 1, you're lvl 16 before that happens. A Barbarian has access to 19 AC at lvl 1 with non-magical half-plate and shield, on top of a d12 hit dice and damage resistance on its own, separate resource. A Fighter or Paladin can have 21 with non-magical plate, shield and defense style.
Beyond the fact that with any smart group melee martials wouldn't get many chances at hitting in the first place, it's a very good floor to hit for a majority of the game, only at level 9-10 can it start being a little insufficient, but by that time party should have sound tactics to channel and control how much melee threat they absorb. Now sure if you let yourself be targeted by more than 3-4 attacks in a single round, "just plain 20 AC" won't be enough for sure. But that's not e mechanic's problem, it's a player's problem.
If Monks could reliably get 20 AC at low levels, you might've had a point here. But they don't without magic items. Magic items that will work even better for other martials, on top of items like +1 armor / shields that Monks uaully don't have a comparable item to.
Of course, AC (and therefor the Dodge action) does have diminishing returns as the CR goes up. Which is even worse for Monks, since they have a d8 and bad saves (prior to lvl 14). Solasta made some homebrew changes to make their lives better, but not enough.
And this is something I tell from experience, in Solasta or otherwise.
I'm inclined to distrust 'experience' when it isn't backed by any data. Confirmation bias is hard to overcome.
Standing still in the middle of a field like you're a scarecrow has never been a smart choice for surviving.
Indeed. But you're not going to get out of a scrap forever, so simply having more survivability for less effort is much, much better if that's what you're looking for in a party slot. Apply the same smart play to another class and not just the monk.
As for the "4 slots party each needs to pull weight" yeah and? If you really want a "tanky" Monk, devs designed an archetype just for that.
It's about the only decent option for Monks we have and what is it? A less useful Barbarian and only manages to get that far after lvl 6. The Barb will do more for less at earlier levels and with better damage to boot. You want to tank? Path of the Stone gives absurd survivability. Had one face-tank a green dragon in the main story on Scavenger difficulty alone for fun. Didn't even need to play smart to win that one. Or Claw for a more balance offense / defense playstyle.
How is any Monk meant to compete with those? Assuming you're not intentionally playing the Barbarian poorly, of course, but even that's going to take a lot of effort to make it worse than Monk.
Otherwise, just be smart. Would you consider a ranged Fighter or Ranger "not pull its weight"? Probably not right? Well then Monk can exactly be that, you'd just miss Archery which is definitely a good boost to accuracy but can also be very well lived without.
You have to build a Fighter pretty well in Solasta for it to pull its weight, considering the alternatives we have. I legitimately don't see one doing it unless you take advantage of the mobility and control options the Spellblade has available. Generally going to do better than Monks though, as long as you're not playing Champion. Well, Champion will do better for a while too.
As for Rangers, it is dead easy to make a Ranger pull their weight. Even if we ignore the very powerful Swift Blade, the Hunter has a decent baseline upgrade to their damage dealing with Colossus Slayer and plenty of useful spells. Fog Cloud, Spike Growth, Silence and Conjure Animals being very strong options included in Solasta. Pass Without Trace got really good after the rebalanced the stealth too. Add in the homebrewed improvements to Favored Enemy (up to +4 dmg per hit) and you can easily justify bringing a Ranger.
Rangers do benefit a lot from defensive feats and anything helping their concentration saves though. Despite being a half-caster, WotC didn't give it much casting support in its class features.
As for the "magic items" problem? Monks have many great ones they can use, besides magic weapons (still useful you don't always want to rely on unarmed), they have also a lot of great items to use, and since they are not interested in armors and shields (although they can still wear Cloak of Displacement and Cloak of Protection which are very good) they have much less competition and choice complexity over attuning to things like Amulet of Health, Bracers of Archery / Defense, Belt of Hill Giant Strength or items allowing to cast spells. So really you have largely enough possibilities in any game.
Armor, shields and most weapons don't take attunement, so there is no competition. Just a straight up advantage to everyone who isn't a Monk. And seeing as +1 and +2 armor is craftable (some are even guaranteed loot drops, like a +1 half-plate), that is a hard hurdle for Monks to overcome.
Your Monk has an Amulet of Health.
The Barbarian has an Amulet of Health and a shield +1. And a half-plate +1. And maybe Punisher, Black Widow or Dragonblade for +2d6 dmg on weapon attacks. Or if those weapons are too late-game, the Frostburn Longsword for +1d8 cold dmg, available around the same time as the guaranteed drop half-plate +1.
Both characters are using only 1 attunement slot.
You could try to force the comparison back to the white room without all these awesome items Monks can't enjoy, but they are pretty common in Solasta's official campaigns.
By the way the average combat has always been around 5 rounds, the "3 rounds average" spouted out by community has always been an illusion and misconception born from the misinterpretation of a few lines of DMG regarding custom monster creation.
If combat had been expected to last no more than 3 rounds on average, offensive spells wouldn't have a 1mn duration, but a 30 sec duration. As simple as that.
If you want to have Monks compete on a 5 round basis rather than a 3 round basis, you just made their lives a lot worse since they don't get any more ki to use.