Hi,
I think your problem is not with LibreELEC, but with the skin itself.
I just got a new RPi4 that is meant to upgrade my actual RPi3+OSMC I have been running for years.
Due to the Kodi 19 situation you mention above, I ended up installing LibreELEC 9.2.6 on it, an it runs great. Faster and with less CPU usage, specially h265, but also the UI.
I read your message and decided to test Aura myself and let you know how it goes in a PRI4 system that has just been built 2 days ago.
And the short answer is: it doesn't.
I followed this instructions to install: Add-on:Aura - Official Kodi Wiki
I did it twice, and it always gets stuck on "Initialising Skin..." If you scape this it moves into "Loading Home menu..." but the home menu is missing items (Movies and TV shows).
When you exit settings, you lose contro and need to reboot LibreELEC via ssh>reboot.
I didn't look any further, but it is clear to me that this skin has problems. MatteN mentioned above it's support has stopped. And things are going to get a lot worse with Kodi 19, as python2 is not supported anymore.
If you do not like Estuary, you will have to find again some other skin your wife likes.
My advice: make sure it doesn't use python2 or at least that they have it in their roadmap. Otherwise you will be in the same situation again when you move to Kodi 19.
I also suggest you stick with 9.2.6 for now, and use a spare SD card to test LibreELEC v10+ before deciding to upgrade.
About your question about the SD cards: I really doubt it will make any difference.
With a fast microSD card, my system boots in 20s and is fast and snappy.
With my slowest microSD card (a card I cannot use for Kali for how slow it is), my system boots in 30s and is fast and snappy.
My understanding is that once booted, LibreELEC loads mostly everything in memory. I personally use my slow card for LibreELEC and save the good one for other tasks on other RPis I have around.
Regards,