SSD reliability

Hello, mb34.

Yes, it seems that SSDs are at least as reliable as HDDs.

See the report at this link, for example:

https://www.backblaze.com/blog/how-reliable-are-ssds/

If you google "SSD reliability" you'll find lots of info on the subject.

I think you'd be fine to do as you are thinking.

Drives for Time Machine should be about 2 ½ to 3 times the size of the drive(s) you're backing up.  I recommend drives from OWC (MacSales.com) as they have a history of reliable hardware, excellent customer support and honoring warranties.  Consider one of these depending on your budget and needs:

Envoy Pro EX with USB-C or USB 3 - best SSD from a speed vs price standpoint

OWC Envoy Pro EX with Thunderbolt 3 - fastest if you have a Thunderbolt 3 port but more expensive

OWC Envoy Pro mini - best for laptops

OWC Mercury Elite Pro 7200 RPM Storage - excellent choice for Time Machine usage, inexpensive and come in larger sizes that SSDs don't.

IMHO, SSDs are overkill for Time Machine. If your external enclosure is rated SATA 6GB/sec (SATA III) and you use a quality rotational hard drive, TM backups are quite fast after the first one.

We exclusively use the OWC Mercury Elite Pro USB3 series containing SATA 6GB, 7200 RPM WD Black drives. No issues. For best reliability and service, get an enclosure that uses 3.5-inch drives and comes with an independent power supply. The computers that earn income each have two OWC drives: one for Time Machine and the second for bootable clones made by Carbon Copy Cloner. Belt and suspenders, ya know.

External SSDs are gross overkill for TM, besides also being less reliable (lower MTBF). Buy a regular flash drive, avoid enclosures and other gimmicks, save money, get value. Can recommend the Toshiba Canvio Premium.