there's a couple subnets I (ab)use in the DOD IP space for my home network knowing they'd never put them on the open internet. it's also fun to throw logging for a loop if someone digs.
22.0.0.0/8 - it's basically free real estate!
I have seen ISP doing CGNAT on DOD IP space.
The CGNAT space (100.64.0.0/10) is also free real estate for container virtual/overlay networks when you don't want to (or can't, thanks to IT) use the RFC-1918 subnets.
I also do this, except 7.0.0.0/8 instead. Its great for not conflicting with hotel wifi dhcp.
the entirety of 10/8 and 172.16/12 and 192.168/16 wasn't large enough for your house?