Running Stalwart in production for ~20 heavily used accounts for some company and no problems so far! The simplicity for such a complex stack and flexibility of deployments is off the charts!
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Never hosted Postfix / Dovecot stack, in fact this is the first time I host emails, but from what I understand Stalwart is designed to handle inbound directly.
For very high throughput inbound you could check out KumaMTA - it was designed specifically for that, but I think Stalwart doesn’t have bottlenecks in it’s clustered topologies which would require it unless you are doing something crazy.
They have very good docs in general IMO, here are docs on how to cluster - https://stalw.art/docs/cluster/configuration