Just some quick thoughts from my first playthrough:
Rowlet was pretty good throughout, if you know and understand its bad matchups then you shouldn't have much trouble. Levels up very quickly and was surprisingly powerful (mine was Adamant) and bulky on the Special side. Lack of moveset hurts a ton as Dartrix, but Leaf Blade and Spirit Shackle are very useful later on. The amount of Dark moves in the game hurts Decidueye immensely. I suppose Upper Mid makes sense to me.
Grubbin ended up being very good for me, and I think the most underrated Pokemon I used. I got it incredibly early on and it quickly became useful with Bite, Bug Bite, and Spark. Grubbin gets Acrobatics, along with Crunch and X-Scissor, and has a ton of options for held item (Electrium Z, Eviolite, no item to boost Acrobatics). It kept up with the rest of my team during this stage and was the best matchup I had more often than I thought. Evolution into Vikavolt is very late, but it becomes incredibly useful with Thunderbolt and Agility. With these two moves alone I took on the Flying Elite 4 member, and it proved useful against the Fighting Elite 4 as well. Its movepool is super diverse, its stats are good enough to get by, it comes super early, as nice STAB and defensive typing is generally good...should be at least mid tier, if not Upper Mid.
Crabrawler was super good early on, but gets so incredibly outclassed later on. Still, Brick Break does a ton of work, especially considering how early on you get a Z move for it, and considering the amount of Normal and Dark types throughout the game. It ends up becoming too slow to work by the end, but the early access and raw power of Brick Break should warrant Lower Mid Tier over Low Tier. It still isn't that good, as even positive matchups are overshadowed by its weaknesses.
Salandit was overall disappointing, as it really didn't have the power or movepool needed to succeed later on. A generally poor defensive typing for the game paired with extreme fragility made it too much of a burden, even with Flamethrower and Sludge Bomb. This, coupled with the difficulty in finding F-Salandit, makes a case for moving it down to Upper Mid. It is still good in the right situations, but it has very little versatility in matchups.
Sandygast was disappointingly slow, to the point that its good matchups were not good simply because it took too much damage. Earth Power and Shore Up come really late in its moveset, and you really have to drag it along in your party until it evolves. Even then the damage output isn't quite there, and neither is the bulk, especially against super effective moves. The best part is probably access to Hypnosis, as well as the amount of great matchups Ground/Ghost typing gives. It completely walled many opponents, and surviving for a turn could set up a Hypnosis to do what you need. It was great for catching Ultra Beasts postgame, but not sure that counts. Mid tier is fine, Lower Mid is probably better.
Wimpod should absolutely move up. It evolves somewhat quickly, and upon getting First Impression it is seriously impressive. It takes huge chunks out of anything, and has survivability with Leech Life and coverage with Brick Break. The bad parts are pretty hindering though. It takes a bit of time to become relevant, it doesn't get access to a Water move for a while, and is practically useless against the Elite 4, in which all of its types resist Bug. I'd say Mid tier would be best, but the uselessness of First Impression was crippling, leading me to pick up...
Solgaleo, who did in fact carry. The only real downside is Sunsteel Strike's PP, but it was super useful and fit onto my particular team well. The prevalence of Dark moves does hurt, but its power is a little silly, with Steel, Psychic, and Electric coverage immediately. High tier seems right to me.
Just some thoughts, I'll be doing a couple more playthroughs and will try to share my thoughts with the stuff I pick up if it is controversial or not talked about enough.