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