I’ve got CM4 no eMCC and want to boot from microSD. Probably will mount a separate home partition from SATA SSD. What speed etc. does the microSD slot on the CM4 adapter support? What specs should I be looking in a microSD for optimal boot drive performance and durability? What brands are recommended for high reliability?

I'm personally using Samsung EVO Plus and SanDisk Ultra cards. Mostly because I trust them and they're durable. They are not the cheapest, though. But really, about any SD card will be ok for a boot/system partition. You do not need much performance off of it.

If the SD was the only storage, system and data partitions, what attributes indicate industrial enterprise grade handling of R/W cycles? Is it viable to run off microSD for very long?

This is going to be hard to find and compare

Thank you! There are so many specs to consider it’s overwhelming.

You could also try the raspikey it's eMMC storage that you can plug in a Micro SD slot

Industrial grade SDcard https://www.kingston.com/en/memory-cards/industrial-grade-microsd-uhs-i-u3 works with Pi400

I personally would say against Kingston

any particular reason?

Yes, issues with multiple SD cards and a with SSD. I don't know about these cards, but lately the quality of Kingston products went down

they advertise it as "industrial grade", just got 1 pcs for testing in hot enviroment and summer is comming.

I'm only sharing my personal experience. Will be interesting to see how it works for you

The articles on the Kingston site are helpful. https://www.kingston.com/en/blog/personal-storage/microsd-sd-memory-card-naming-conventions

i didnt see before (i didnt look in details) other SDcards have Wear levelling and Garbage collection. For any flash device used in desktop or server this is important for perfomance and endurance. Write amplification can be tunend with file system.

@geerlingguy also published several posts on his blog about different microSD cards, their specs, and what was genuinely useful, etc.