Here's a frame-challenge - do you need a strength stat at all? Game controllers are designed to be require as little physical effort as possible to read inputs accurately and satisfyingly, so there aren't going to be a lot of obvious ways to map real-life strength to in-game strength. That said, I do have some ideas below, with caveats.
But rather than struggle against the limitations of your idea, why not embrace them? If there isn't a compelling reason to include strength in your game beside "other RPG games use strength" you can absolutely go without it. If the idea of minigames that test skills that correspond to in-game stats is core to your game, that should take precedence over other considerations that aren't as core.
So maybe your world outlawed big beefy swordsman, and instead everyone is a wizard or a ranger. Or maybe there are gigantic anime-style swords, but they're magically enchanted to not require any particular strength, and instead are all about hand-eye coordination and reaction speed. There are endless other reasons a world might lack strength-based fighters, but whichever one you come up with, you have some interesting opportunities to differentiate your world from those in other RPGs.
All that said, if strength is a core part of your game idea, what I've seen work in other games for similar purposes is button mashing (or some extension thereof). It works thematically since it's brute-force-y and can be a bit of a workout for your wrist muscles. It also doesn't have to be limited to literally mashing a button - for example, you could spin a joystick really quickly, or rapidly alternate between pressing the left trigger and the right trigger.
It has some disadvantages, though: it's hard to generalize cross-platform, since things that work well on console like spinning the joystick don't work on PC or mobile, and vice versa. It can also run the risk of players damaging their controllers or even themselves, but you should be fine as long as you don't go overboard on how fast people need to go.
