More compression (smaller files) is the only reason to choose a higher V-number.
Mp4Als, because it is so similar to TAK, but can use up to 1024 predictors (if i remember this right) compared to TAK's 160. I found 16 files where Mp4Als is at least 0.5 percent stronger than TAK. In any case there is also a clear advantage of TAK's preset 4 over 3, which are using 160 vs. 80 predictors. Imho strong evidence that the advantage of Mp4Als is based upon the higher predictor count.
You are right, TAK does not by default.Also offering the fastest encoder, with -p0, but we knew that.Of note when comparing FLAC and TAK: in this comparison FLAC does MD5 summing (as it does this by default) and TAK does not (as it does by default), if I'm not mistaken. I didn't check, so if default behaviour of TAK recently changed, this is untrue. Calculating an MD5sum at these speeds is about 30% of the work, so it makes quite a difference.
because it's not reliable enough as you yourself sayThat's the most reliable way. If it's not enough for you then use lossless.
I use the feature & in my case there are duplicates between the folders & I didn't want them merged. I checked a few discogs vs lfm ones and there seemed to be some duplication of images in those two sources as well. You can always merge them yourself into the lastfm download folder. I can understand you might not want to do that, but at least there is a solution. If you decide to try, I suggest using a back-up copy in case it doesn't work out.
...possible to merge the artist photos