my previous b450 mobo was using it for a few years without flashing the bios. post speed was all quite standard for me no issues. only after this problem that i flashed with the flashback button but my flash has all been smooth without issue. the light blinks then speeds up then slows down then when its done it will shut off itself and restart. all in a few minutes. even for this new b550m board i also went to flash it to the latest stable bios after i the post failed and it flashed the same way successfully too.Has to be the CPU now as you have tested and changed everything. Do keep us updated on the progress.
Your PC could have been a good sample for Greg Salazar's youtube videos.
MSI motherboards do take a longer time to post. I am using MSI board too and it post slower than my previous ASUS board. Something is weird for my current MSI board which is the bios flashback button does not work. The light flashes for a few seconds and then ends. I went into the bios and sure enough the bios wasn't updated from the flashback button. Lazy to rma the board just for that as I could update it from the bios. Not sure if it happens in your case too.
maybe you wanna check if you using the right usb 2.0 port. cause for my b450 the backplate shows which usb port is for bios flash while the b550m doesnt so need to check the manual for the right port to use.