You may need an active TB3/4 cable. 30 cm is typically the longest passive cable you’ll see. Active cables are more expensive, though less so than they used to be, and can handle longer runs like 3 meters.

My apologies, I missed a few of your questions at the end.

• Yes, any USB4 rated cable can carry the TB protocol at whatever distance it’s rated for, otherwise it isn’t USB4.
• Likewise any TB3 cable should work for your application, if it’s actually TB3 rated.
• 3m is generally the max length I’ve seen. The theoretical limit may be higher but I suspect latency of the cable itself becomes a problem for PCIe tunneling, which is still a synchronous interface TMK.
• The TB controllers decide whether your cable is sufficient. They will negotiate a link if they can both verify a nominal link speed/multiple. So the lower rated USB cables won’t work for PCIe tunneling, even if your ultimate bandwidth requirements are minimal.

A few things to note if you’re shopping on places like AliExpress, eBay, Amazon, etc:

• Be wary of any long (active) TB/USB cable that’s cheaper than $30.
• Be wary of any cable that has a small connector, like one you would use to charge your phone, because the TB/USB4 connector itself must house an active signal repeater chip, making them chunkier and/or longer than usual.
• Similarly, the shielding requirements are fairly substantial, so the cable itself should be beefier than most USB cables.
• IME generally, PCIe over TB/USB can sometimes just be finicky. Of the 12 or 13 cables I’ve used with various combinations of machines, enclosures, docks and risers, there are occasionally some combinations that just don’t play nice, for whatever reason.

FWIW I don't think you just lose bandwidth with longer cables, but rather it just loses sync and the cable stops working.

I have a thunderbolt 4 hub that I wanted to tuck away somewhere, so I tried several longer cables (some usb4 as you said, one actual longer TB cable) none of them worked reliably like the 30cm cable that came with it. It would sporadically lose connection for a couple of seconds before reconnecting in most cases or it just wouldn't connect at all.

Not sure if my hub or cable choices are the problem or if it really does just have to be as short as possible in some cases.