If you want to be bored silly, search for my old posts on media playing issues in my 2015 Limited. You seem to have a 2018 Subaru, so my experience may not be relevant for you.
Reindexing at every start up is a problem. I keep 20G or 4000 songs on an SD car with USB adapter. It's a fast card. But reindexing takes forever seems like about 30 min. ...
“Reindexing” (as you call it) is indeed a problem on the 2015 Limited. I use fast flash drives with about 60GB of music and about 9,000 or so tracks; original indexing takes 15-25 minutes, but the “reindexing” on later car startups is more like 3-5 minutes. Possibly your SD card isn’t as fast as you think, or your file and folder structure is overly complicated. In general, I have one top-level folder for each album artist; within each of these folders, I have one folder per album. Track artist, album artist, album, title, and genre tags are completely populated in my music files.
...Also either I missed it or there appears to be no method to play an album within artist. ...
This doesn’t sound right. On my 2015 Limited, I can choose to browse music by “artist” and then within artist by album. I find this “artist” approach useless, because the media player uses the “track artist” tag rather than the “album artist” tag, but it does work (although when you turn the car off and back on again, it reverts to its default folder-and-file order). I’ve read somewhere that Android generally ignores the album artist tag, too, so my head unit is not alone in this lunacy.
Do you have the “album” tag filled on all of your tracks?
... Furthermore if you play by albums either it's me or it doesn't play an album in track order, it lists songs alphabetically. I can cheat and insert a track number in the title but then indexing takes even longer indexing probably because all the songs are starting with 01 then 02 so that have similar key values (titles)....
On my 2015 Limited, the default “order” in which songs will be played within a folder (which on my flash drives is more or less equivalent to by album) is the order in which they appear in the flash drive’s file system for that folder. This will typically be the order in which they were copied to the given folder, which will often be alphabetical order by file name (depending on how the copying was done). I often put track numbers at the beginning of file names (not the track titles). It never occurred to me that this might affect indexing times.