It sounds like you’re looking for intrusion detection or prevention rather than antivirus.
Great list here https://github.com/sbilly/awesome-security#ids--ips--host-ids--host-ips
However it’s likely that these do not run on asuswrt-merlin out of the box and may require additional setup or hardware.
You may even be good enough by using the built in firewall on the router interface and making sure no ports are open/forwarded.
But, why?
ClamAV is a scanning engine that uses a database and heuristics to detect an infection.
Are you looking for an Anti Virus Scanner, or Intrusion Prevention System?
is it foss ?
4.01Mb may be too small for a single block.
Usually, the larger the drive, the less $ per GB. 16 and 18 TB drives for example would be where you would want to aim for if you’re starting out fresh.
If you want to mix and match what you already have, unRaid is an option that does that well.
One of these should do what you’re looking for. Each has a slightly different approach.
https://etcher.balena.io/
https://unetbootin.github.io/
https://www.ventoy.net/en/index.html