If that order is correct that would mean that incorrect results are highly likely, as dns updates might never make it into the cache. (EDIT: did a quick test with a 15min TTL on a temporary external subdomain, it was correctly updated an denoted as "OK (forwarded) SECURE" in pi-hole, so everything should be ok).
Been doing some benchmarking over the past day, using 31k domains with the folowing command: time dig -f domains.list +noall +answer > /dev/null
Using redis it takes roughly 28minutes and a few seconds to resolve them. Without redis this takes about 33,5 minutes.
While using singular queries it's way to fast either way to leave the noise margin, so in the big picture there appears to be a benefit.