I have a JOBO 2500-series tank which I use whenever I have lots of negatives to develop. Using it by inversion is not really a problem, as I generally use highly dilute developer. 1.5 liters is not too much for 6 sheets...

Most of my processing is done in trays. I have discovered that my darkroom isn't quite dark, so that I can generally see the outline of my hands after six minutes or so. Then it's just waiting for the first traces of highlights to be visible on the sheets... It's possible I get some fogging this way, but I have not been able to see (or measure) any difference in base+fog on tray-developed versus tank-developed negatives. So far. But then again I've used nothing faster than FP4+ so far, it may be a different matter with HP5+...