I'd personally recommend AdGuard Home over PiHole, as it supports DNS-over-HTTPS and DNS-over-TLS out-of-the-box.
Honestly, if I was doing this, I'd probably run it on a VPS or my home server. I'd run at least two instances and configure their router to use them. You'd lose encrypted DNS, but it means they don't need a device on their local network.
I would like to collect some suggestions on how to deploy such a box
Someone will have to do maintenance eventually. Configure SSH securely (Ed25519 key, password authentication disabled), and SSH in and update it once per month? Charge them a small fee, or do it for free in exchange for them doing something nice for you.
Auto-upgrades are scary since there can be breaking changes between versions.
My hardware platform of choice would be a cheap thin client
Why not a Raspberry Pi? The supply chain issues are clearing up.
You may want two so that there's redundancy and they don't lose DNS in case one dies. You can use something like https://github.com/bakito/adguardhome-sync (or the PiHole equivalent) to keep configs in sync across both.