YouTube blocks it. There are extensions to allow it (like Vinegar) but by default it’s blocked. Brave might work around YouTube’s block in the same way.

This sounds like you’ll need to do a balance operation. Try this first and see if it helps:

btrfs balance start -dusage=0 -musage=0 /

If not you can increase the number to 5 or 10. This operation reallocates chunks on the disk and ensures they’re filled - check https://btrfs.readthedocs.io/en/latest/Balance.html for details.

I’d have to check my iptables syntax again but I’m not sure you want the FORWARD between the networks unless C has a manual route to get traffic for the 192.168.15.0/24 network back via B. You just want to NAT A behind B’s IP on 192.168.38.0/24. I think the forwards are sending the traffic without doing NAT on A.