Trying to play perma offclass is objectively an inferior playstyle, because if it wasn't, there are definitely a lot of higher level teams in invite/prem that would do it.
If you really care about trying to test the viability of other classes (and not just trying to convince yourself that full time pyro/spy works because they're your favorite classes), then you should be looking at the opportunities where temporarily switching to a different class would be more beneficial then playing the stock lineup.
The obvious example is running engineer on last, because a well positioned level 3 sentry gun is really good at wasting the enemy's ammo and shooting people who hold w too much, but offclasses don't have to just be on last. I remember a few season ago a lot of ETF2L teams would run sniper between mid and second (until I believe some of the angles were changed to favor sniper less).
Things like this should be what people who care a lot about offclassing, should be looking for. Find situations where pushing a specific last with pyro or heavy, would be more beneficial if your team plays around it, or a mini sentry engie holding this specific flank makes it extremely difficult for the enemy to punish any aggressive play.