Get a 06 or newer YZ250 and it's basically the same bike as a 2018. If you want to tame it down a little buy a flywheel weight for $100. Awesome bikes that can do pretty much anything from tight single track to motocross. Parts are easy to find and really easy bikes to work on.
If you wanted a trials bike you would already know it.
The thing is that most people don't know what they don't know. For example i started on a 4 stroke, what a mistake that was, but unless i had made the mistake I wouldn't have known. Trial bikes is probably real good advice but the problem is most people already have their mind made up and most people don't know much about trials when in reality is probably a really good option.