I was going to come in and say pihole, hands down as it got me into home labbing. But then I got reading some of the responses and my thoughts changed to Home Assistant because I can automate so much. An app that I had purchased to help me stay on top of my medications decided to go subscription so I was able to use HA to set up reminders to take my drugs go off on my watch which I would turn into reminders to do so much more. Than I thought Docker, because it is so much easier to install things with docker. Than I thought OpenWRT because I now have a real firewall that I can do so much more with than my ISPs dinky little thing. I was almost ready to reply when I read your question over again and I saw you mention the *arr stack and Jellyfin, which got me thinking how much I love that. Which reminded me of someone saying immich. I have to go for some blood letting today so I keep my blood req in paperless. I self host nextcloud and that is one thing I am looking at finding alternatives to but all that being said picking one is like picking a favourite family member, sure some you may like more than others but you are never going to tell which one.