I don’t use my BMX often enough, last time I visited a track and had a pootle I was told I had “a good pump technique” whether this was a genuine complement,a veiled criticism of my less airborne riding style or a covert way of calling me a **** I’m not sure…
BMX riding does feed back to your MTB technique, but takes adjustment as better rolling 26” wheels and suspension forks seem to make you into a total Jessy when you get on a diddy bike again…
21” TT – GT Fueler here, basic cheap, slightly heavier, Dirt BMX – does the job, very noisy cassette hub, I always intended to get out and do the odd bit of racing on it but have yet to do so… don’t spend a fortune unless you’re properly serious…
I did have a 24” MTB for a while, that was good, MTB geometry, layout, and bouncy forks, but not as robust or cheap as a BMX or Cruiser…
A cruiser could be a good in between option for you, not such a drastic change in wheel size so better rolling, but still the same basic handling of a BMX…
Good luck… hope your hunt goes well…