There are also mail sending services like mailgun on which you can define inbound rules, like a catchall+forward rule, on domains or subdomains. I do use it for a few domains, although my "main" mails are managed using gsuite+domain aliases.

Just use webmaster tools to know what 404 googlebot finds and add redirects to important ones. You'll only handle the 404 that crawlers know about, which should be enough.

Don't consider ES bellow at the very least 1GB of data, and don't consider Hadoop and friends bellow at the very very least 1TB of data.

5000 ? Why not an in memory hash ?

If you wonder, then probably not worth it for you. Otherwise and without having an opinion about which implementation is better, having a standard way to make your services talk together is worth it, given that you have enough services to make the additional work neglictible compared to the time it saves.

From a solopreneur / indie point of view, probably not worth it. Remember that GAFA builds software for GAFA-scale engineering.

For #apercite, we use : Drift (live chat), UptimeRobot (status page), Sentry (error monitoring), CloudFront (CDN). We also have Prometheus+Alertmanager+Grafana for internal monitoring, although we host it ourselves. Slack is our communication central, most of those tools post messages in channels there if anything requires our attention.

You probably won't get a good one for free. I think scrappinghub has a (paid) service for that (I am considering it for #apercite spiders).