The LPSU makes a noticeable difference - the sound has more body without being flabby. Even my wife (who says lossy streaming from Spotify is fine) noticed the difference almost immediately. Whether it’s worth the extra outlay is another question - that only you can judge. The same for an external DAC - I’ve been using the ZenMini as exclusively digital out so can’t say what I think of the internal DAC.
Re transferring your Roon database over to the ZenMini, do you mean all your FLAC/WAV files? If so, importing via USB on the ZenMini is straight-forward - it’ll take a long time with any sizeable library but I’ve done it loads of time without any hiccups. The only potential headache is missing meta data which, if you’re ripping to FLAC CD by CD, is easier to spot as you go. That said, I think the ZenMini failed to recognise less than 10 CDs out of 1,000 so this is an area where it excels compared to other products on the market.