I suspect that those jobs worth chasing that pay proper, they expect and will train/test you to their standards whether you’ve already been doing the job.
Proper car transporter operators do this regardless, been off for injury long term and definately with new starters even if long experienced, they’ll put you with a trainer to make sure you’re up to speed and doing it their way, no different on tanks, yes you’ll get a compressed re-training schedule but you will be trained and have to show the trainer you can do the job to their standards.
I certainly wouldn’t bother doing ADR or learning or providing any other skills to the company for free, it would either pay me to do so, or maybe at a push if it got me in at a more normal job literally a stroll from my gaff so paid me in another way, ie one car less zero travelling costs.