Once you’ve got you initial DCPC you’ll eventually be needing to think about renewing it with periodic training. If you did the full standard ADR course you should be able to get 28 hours of the 35 hours you need for your next card.
Whereabouts you’re based is obviously a major issue. But, if you were in my neck of the woods I’d say do it, we have a lot of ADR near me, plenty of work for both Class 1 and Class 2, you can get a full ADR inc Tanks and DCPC hours for £450, so subtract the cost of four days DCPC (lots of variation there obviously) and it would only take one week of ADR agency work for the course to have paid for itself.
HTH