Running Pi-Hole and a VPN on your router might not be a good idea.

For Pi-Hole the reason is that it runs all it’s components as the root account. One of them includes the PHP webserver for the dashboard.
On a more complete Linux system like raspbian, confining it to a container helps somewhat by limiting what can it do. But I’m not sure if you can run a container environment on OpenWRT. Also, routers are often quite week, so it depends on your model whether pihole would fit in to RAM at all, with all your filter lists.

For a VPN the reason is rather just the performance.
On one hand the VPN may be slow, and on the other hand it could slow down the router while someone is using it, which would affect all traffic that needs to be filtered or routed (to a different network, including the internet) from your home network.

I’m not a pro, so take these with a grain of salt, and verify if these are actually true.

We should report these as advertisement. Afaik admins too receive the reports and can act on it, in case the mods are away.

I’m not quite sure, but I think the majority of the devices in that price range, if not all, fall into that category, being made with 4MB flash memory, and 32MB RAM. OpenWRT can hardly fit into that, it’s only possible with a custom image and making compromises

Do those have any memory (storage and ram) to run a secure, up to date firmware, like openwrt?