Do you need a TPU? If you have a 7th gen or later Intel iGPU you can use OpenVINO in frigate and it works just as well if not better.

I don't really want to use a USB TPU (no real reason)

Since there is no real reason, why no reevaluate?

I connected my USB TPU to a spare internal USB 3.0 header and mounted it inside the case. Frigate has been working perfectly fine for years.

I also use quite a few PCIe M.2 adapters but only for SSDs. No complaints here, full transfer speed and reliable. No reason why it shouldn't work for TPUs.

I use a ton of pcie adapters at work, but not really sure what you are doing? Pcie to m.2? Almost all the ones I've used off of amazon have worked fine except a sabrent one, it worked fine for a m.2 drive to have an os but when I used it for testing drives it didn't work, it also took up a whole x16 slot for no raisin

We use ones that look like this

I'd have to dig through my Amazon orders, but I ran one such adapter in my work desktop for a while.

Just read the 3 and 4 star reviews and go from there