Your feat choices are extremely suspect in my opinion. Why are you not taking either of the feats that best take advantage of your Strength, Tavern Brawler or Great Weapon Master? Either of them would be a huge damage boost, for thrown weapons (Tavern Brawler) giving you some great ranged options, or for normal melee hits when your hit chance is good enough (like say if your enemy is paralyzed and you're auto-critting anyway)? Athlete, Alert, and especially Mobile seem like marginal benefits by comparison.
Damage output is already significantly high as a Fighter, and while I could certainly double down on it, there are a couple layers I had to consider. Not every fight and challenge is the same, and as a solo player with very low HP, doing hit and runs is almost mandatory to avoid taking a lot of unneccesary damage - I wouldn't want to waste Bonus Actions on healing or take more Short Rests as Long Rest costs 80 supply on Tactician. On top of this, some mobs have WAY more HP than others and enemy Crit RNG is turned to the Heavens on Tactician (at least it feels like that, I barely ever got critted on Explorer or Balanced, but I get constantly critted on Tactician), so I prefer to have more consistent options of avoiding damage. Athlete and Mobile help with this quite a bit (not to mention Athlete helps a lot with exploring the world or getting to places where you can abuse the enemy hard), and Alert simply ensures that I'm always on the advantage, without the need to savescum for Initiative rolls. With the heightened DEX values and the Caustic Ring, ranged weapons are no slouches either. And my final argument for skipping Great Weapon Master is simply that as DU, you get to benefit from your Invis Cloak way too much. You really don't need the extra actions after a kill when you turn invisible and then re-open with Surprise once Combat drops.
Tl;dr: Big damage feats aren't as consistent as what I'm looking for.
Also, you're missing a few stat setting items, most notably the headband of intellect from Act 1 or the Gauntlets of Hill Giant Strength from Act 3. Granted that you can't be using Dex and Str boosting gloves the entire time, but since you don't need to be focusing on Int you can boost Dex, Wis, and Cha and use the Str gloves once you have them. That ties into a general principle, you don't seem to be leveraging respeccing nearly as much as you could or imho should when doing a solo run - it's only 100 Gold, doesn't mess with companions, lets you kit yourself out for a given encounter or scenario, and overall lets you adjust to the itemization you currently possess, such as dumping Dex once you get those gloves in Act 1, and in Act 3 shifting to increase it once you replace those with the gloves of Str.
The INT Hat would conflict with Grymskull and Helldusk Helmets, both important mitigation pieces. While I could look to minmax my stats more that way, I can't put more points into WIS and CHA, so I'd be forced to put points into the stats I am already looking to get items / effects for - STR, DEX and CON, essentially ending up with worse gains by using more.
I didn't mention the Gauntlets of Hill Giant Strength because it only sets STR to 23, whereas Cloud Giant sets it to 27. Given you could use other Elixirs if you go down that route, but you also lose all your DEX, eliminating Ranged weapons as a viable damage source barring Celestial Haste from Gontr Mael.
And I probably should have mentioned this, but this build was the result of constant respeccing. I could multi-class for sure, but as the title implies, this is purely for the Fighter, and once I figured out what I wanted to do, I've gone from start to finish without respeccing once. Imho, it also makes the flow of the game better, and makes hitting milestones more satisfying in my book.
