best TB5 ssd for my laptop?

It's difficult to say what the "fastest" is because such devices are expensive. Few people have them. The few that do have only one.

But generally speaking, what you want is a Thunderbolt enclosure with a very fast M.2 chip inside.

Here's one such enclosure: https://ca.macsales.com/shop/owc-express-1m2-80g

And here's an M.2 chip: https://ca.macsales.com/item/OWC/SP4P4T1AT02/

There might be other combinations that work too.

why would you need a Thunderbolt-5 device when most folks are perfectly happy with more affordable devices?