Hi it works but I hit the 500k limit in no time. I am dead sure however this is because of the language detection kicking in on a lot of old but still relevant content as indicated as a caveated by @tgxworld.
Now I was wondering. If I switch to say the libre translation service, get all the language detection sorted that way and then would switch back? Would that work? Does the language attribute of a post get retained after switching a service provider?
The quality of deepl is great, and the limit should work in BAU mode. Just need to cross that language detection river somehow.
Other options would be pay for the paid plan for a little while, but would do need to do a character count of the whole site. Depending on that count being patient for a couple of months might also work provided it picks up where it left off when it hit the api limit.
I will investigate and report back.