A quick tip: be sure to reset the PRAM with command-alt-p-r holding down during restart until you hear 3 chimes. Then while the machine is booting hold down command-alt-o-f and type "reset-nvram" and then "set-defaults" and then "reset-all" (all of this is in open firmware) before installing Mac OS 9 to make sure firmware is in its original state.
I came across this idea of SSD upgrading and installing Mac OS 9 in April 2024 and bought three broken ones to build one for my son. [0] When the first one worked, I ended up figuring, why not just finish the other two and sell them on eBay?
That led me into a hobby business. I've now cleaned, upgraded with SSDs, and sold about 70 of them. The "business" basically breaks even, so it truly is a hobby. In fact I invested so much in inventory buying 90 of them in a lot at the end of 2024 that I am negative right now. I will probably turn a slight profit in 2025. But it's fun and if you want you can buy one from me at: https://os9.shop
Sorry for the self-promotion, but very relevant!