Dokku is great, but historically it didn't really handle resilience. It looks like there's now a K3s scheduler (added earlier this year) which would mean I could have use a Kubernetes operator for a replicated database as well as have the app running on multiple boxes (in case one fails). It looks like it'll even setup K3s for you. The docs don't seem to go into it, but hopefully the ingress can also be setup on multiple boxes (I wonder if it uses a NodePort or the host network).
I was sad when Flynn died (https://github.com/flynn/flynn), but it's great to see Dokku doing well.