Been trying both Mail Archiver X and Horcrux. I landed on the latter, as Mail Archiver X seemed to be quite a resource hog and – critical for me at the time – would not be very considerate of the main drive space, which is a huge issue with a small SSD.
Horcrux is leaner (even if it could be even leaner to my taste…) and does its job well and silently. I’m happy with it, thanks for the recommendation. 