Yeah, utterly Level 5 is like still a beginner adventurerer. Yes, they are not total noobs anymore, but they just graduated from slaying giant rats and baby dragons. I don't think a lot of people have a problem with Wizards and fighter at Level 5. It's the Levels above 12 that make problems.
Yeah, at 5th it's still arguably okay. I do think the gap widens significantly over the course of T2, with spells like Polymorph, and Wall of Force. Fighters would benefit a moderate boost in T2, but primarily in terms of utility, IMO.
It's really in T3 where that gap widens to a chasm, with spells like Mass Suggestion, Simulacrum, and Mind Blank. Fighters get their 3rd attack (roughly a 50% damage increase), a second use of indomitable, and some ASIs. That's nowhere close to on par. The wizard becomes effectively immune to all mind effects, and the fighter gets to take a feat that's balanced for level 1 characters?
Depending on the GMs generosity with magic items, that gap can be narrowed noticably, but IMO that is a terrible solution. Some GMs don't like to give out magic items. Some don't like handing out bespoke items, so what you end up with could be completely random and not especially helpful. Even a GM who tries to give out beneficial items, may not understand what the character actually needs. I've seen this plenty of times in play, where the GM gives out a custom built item that they think is perfect for that character, and then it sits unused in the character's inventory for the rest of the game.
If the game expects martials to have magic items to make up the gap, the tools for acquiring them ought to be built into the class. It would be like if the wizard didn't automatically gain new spells as they leveled, but had to find them. That could make the wizard pretty bad, depending on the game they were in and whether they ever found any decent spells.