I can recommend some stuff I’ve been using myself :
Dolibarr as an ERP + CRM : requires some work to configure initially. As most (if not all) features are disabled by default, it requires enabling them based on what you need. It also has a marketplace with a bunch of modules you can buy
Gitea to manage codebases for customer projects. It can also do CI but I’ve not looked into it yet
Prometheus and its ecosystem (mostly promtail and grafana) for monitoring and alerting
docker mail server : makes it quite easy to self host a full mail server. The guides in their doc made it painless for me to configure dmarc/SPF/other stuff that make e-mail notoriously hard to host
Cal.com as a self hostable alternative to calendly
Authentik for single sign-on and centralized permission management
plausible for lightweight analytics
a mix of wireguard, iptables and nginx to basically achieve the same as cloudflare proxying and tunnels
I design, deploy and maintain such infrastructures for my own customers, so feel free to DM me with more details about your business if you need help with this