I just did this a few weeks back to an old 5,1 I had lying around.

https://www.amazon.com/gp/product/B07FN3YZ8P/ref=ppx_yo_dt_b_asin_title_o05_s01?ie=UTF8&psc=1

with

https://www.amazon.com/gp/product/B07BN217QG/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

Boot raw from a "Install Mojave" USB stick. I'm using a Radeon HD 7950 though so that may make things different.

Here is the disk speed :: https://i.imgur.com/KjPVdFO.png

Yes, the external one would be connected by USB, limiting the speed.

You would get sweet Samsung Evo 1TB :)

https://www.amazon.com/Samsung-Inch-Internal-MZ-76E1T0B-AM/dp/B078DPCY3T/ref=sr_1_1?dchild=1&keywords=samsung+870+evo+1tb&qid=1592593400&sr=8-1

Could get M.2 version for a little bit more, but you need M.2 slot on your motherboard.

https://www.amazon.com/Samsung-970-EVO-1TB-MZ-V7E1T0BW/dp/B07BN217QG/ref=sr_1_4?dchild=1&keywords=samsung+870+evo+plus+1tb&qid=1592593421&sr=8-4

ADATA drives also got nice quality/price ratio :)

Yup it will work. That SSD is form factor M.2 (2280), which the motherboard has a slot for. FYI the regular EVO will work as well Incase you’re looking to save some money.