Ah that is what I feared, sounds like the Flea is just going to frustrate and annoy more than provide a platform to learn on then. The one I've seen isn't even all that cheap (it's Flea 4 so comes with the higher spec) but I suspect the seller will take an offer.
I'd say I want either 26" or 24". Many moons ago I had a go on a friends 20" mod bike (IIRC it was an Onza T-Bird) and I didn't like it at all, I think years of MTB mean I want something that feels like a "normal" bike. I had a Giant STP a long time ago which I now really regret selling, despite not being a trials bike it did well at a bit of everything. And Jeff Lenosky never had any trouble making them look capable.
Inspired Flow XP seems like the best value new bike, but we're already into silly money at that point. A Hex popped up very local to me for a reasonable price, but the listing vanished a day later so expect it got snapped up. I remember the Onza Zoot used to be a good mid priced option? Looks like they're aren't many of them around these days either.