So, according to what I posted before and you picking Rotom-W because that is what your team needed which apparently not.
Either way, looking at your team as a whole it lacks types compatibility and too many mega potentials, you need more diverse coverage.
1st, you would apparently mega the D-Venusaur if you are gonna keep it, making that all other mega potentials in your team mostly not viable. (If not, other options can also be discussed later.)
Typlosion is out and can be replaced by either Talonflame, Blazkin/Infernape, or Volcarona.
Here you got 2 options:
keep Rotom-W and replace Swampert by either Garchomp, Mamoswine, Gliscor, Hippowdon, Delta Muk or Excadrill.
keep Sawmpert and replace Rotom-W by either Magnezone, Jolteon, Thundurus, Galventula, Elektross, or Heliolisk.
Flygon is out and can be replaced by either Tyranitar, Weavile, Serperior, Bisharp, Scizor, or Ferrothron.
When you pick a Pokemon make sure no 2 have the same typing at the least, and different coverage from bulk to sweeping, move-pool and abilities utilization.
Honestly, I never researched so I didn’t know most of my team have Megas
Where can I get Infernape or that Blazkin? (probably that is Blaziken).
I’ll probably keep Swampert since he does more than Rotom W.
Isn’t Thundurus a legendary or am I stupid? I’ll probably choose Elektross since it has levitate as well (where can I get Tynamo tho?)
I am thinking of choosing Tyranitar (and probably buy that armor) but I am thinking of Weavile too. i’ll mostly choose the one who have the best move pools
Where can I get Infernape or that Blazkin? (probably that is Blaziken).
You can acquire either from the Friend Safari. Make sure they got their Hidden Ability, otherwise don’t use them.
I’ll probably keep Swampert since he does more than Rotom W.
Isn’t Thundurus a legendary or am I stupid? I’ll probably choose
Elektross since it has levitate as well (where can I get Tynamo tho?)
In my opinion, I’d favor Rotom-W over Swampert, but that is up to you to decided not me.
Also, Thundurus is a Legendary Pokemon right, but that doesn’t mean that you can’t use it. He is actually pretty much reliable. And Eelektrik can also be found within the Friend Safari.
I am thinking of choosing Tyranitar (and probably buy that armor) but I
am thinking of Weavile too. i’ll mostly choose the one who have the best
move pools
By the current team setup anything from the last choices are valid, so pick your liking.
If you are around breeding, then some can get good moves when breeding. But that is up to you, still can be usable.