Actually it looks like Musicolet can't be blamed for anything. As usual it is in the files. The arts not showing also aren't showing in Windows, though they show in mp3tag and iTunes. Maybe after I go thru and re-embed the missing arts with iTunes as a 'whole album' approach the files will be their best ever (I say this every year). Still though that scoped issue.
There may be some app out there that has what Musicolet USE to have which is instant adding of all music files in the designated music folder to the library in the app, and instant flawless recognition of the entire giant library (within 1 minute) on initial launch. That was the coolest. Biggest problem with the Musicolet (now) and something I just don't understand, is how it takes days (just last week it took 2 days to add 2 albums despite I keep my phone on all day) on my Samsung running Android 13 and only moments on my Sony 1 ii running Android 12 to recognize a newly added mp3 file. I think Sony was ahead of the game by not pushing out Google's Android 13. I don't know though.
Well apparently that depends on the phone. Has been a problem in my Samsungs for a few years. On the Sony it may take long to load but I can't remember the last time loading the whole library there. It loads the new added albums within a minute or so. I am never sure that there is not a problem with Musicolet. I just figure it's Scoped storage causing it, it started happening around 3 years ago.
The best suggestion I can make here is to stop using iTunes to do anything other than sharing your library to other iDevices you may own. And for the record, I have been using iTunes myself since the day it launched for Windows, and have four iPhone users and multiple AppleTVs in my house. So it definitely has a purpose.
While iTunes is (still) free to use, it really is a case of a jack-of-all-trades/master-of-none product. It can manage some metadata, but none of it well. It has a basic filter for searching for artists/albums/songs, but these cannot be combined or enhanced. It can rip CDs but has very few choices with regards to formats and options. It can convert between some of those formats it supports, but not all. And for file management, well that is just not it's strong suit at all.
Stick with what you know works best. In the case of managing your metadata, let mp3tag be your champion. For ripping CDs, go with a reliable app that is designed just for that, try EAC (free) or dbPoweramp (paid) that do this well. Let iTunes just manage moving these tracks to your devices, and maybe be the player on your local machine.
And as far as converting between formats, or even as you have recently stated mp3->mp3, there is zero benefit to this. In fact converting anything lossy (mp3/wma/aac/etc) to any similar lossy format only introduces more issues and loss. Only when converting between the lossless formats (FLAC/ALAC/WAV/AIFF/etc) can you get away without introducing more compression and sacrificing further quality.
This is beyond the scope of what mp3tag is for. I would spend more time seeking support from those player apps, or even from Samsung if you feel it is device dependent. Power management is often the #1 culprit and there are methods to disable that. Accessing file storage, and avoiding using the root level is also high on the list. I have a 1TB SanDisk card I use on my Android device, and a fresh full rescan of 26k songs takes under 10 minutes on any of the 4 apps I currently have installed - GoneMAD, Neutron, Poweramp, and Foobar Mobile.