Woah there - 2.5 on a sub20mm internal rim - SQUIRREL CITY.
I would be VERY Cautious about doing anything other than a slow trip to the shops on a 2.5 on an 18mm internal rim. I push it by putting 2.35 but even that can be a scary ride on roots descents and fast berms. Max 2.20 in my view.
It’s a weird project... I ride big tyres on most of my mountain bikes. They provide a nice comfy ride. I’ve narrow tyred bikes if I want to cover more miles..
You can fit as wide as will fit as long as you accept the handling compromises and fill them with enough air. I have fitted 26 X 3.0 before but pump them very hard otherwise handling is awful and tyres can roll off beads when cornering.