This is part of the problem as well. Some people will have an 8GB card of XYZ brand and it will work, and for some it won’t. The problem is that while the outward appearance or even model number of the card may be the same, the internals may not. This makes it even more difficult to discern problems.
I am confident enough that I would bet that every affected user has either a Transcend or Kingston SD card. They used to be good, but I noticed over time new SD cards would have a severe deterioration in random 4K write speeds, despite the model number and outer markings on the card being the same.
Another user has asked for my opinions on SD cards, which I have written about here: SDCard Choice - #2 by sam_nazarko
If you keep dtoverlay=mmc in your config.txt, your system will be preserved across updates, but you will not be enjoying the benefits of the new SDHost driver. It may be a good idea next update to try removing the line and seeing if the system still boots, as there may be some improvements to the driver that make your card usable again.
I will leave this thread open for a brief while until closing it as resolved.
Sam