For pihole you don’t need support from router. It’s convenient if you can adjust dhcp-server settings so that pihole will automatically cover your whole network, but it’s not a requirement, you can just manually set each device to use pihole as DNS server. All you need is a static IP address outside your DHCP -pool. For spesific router configurations, you can ask those too, just include spesific model and possibly screenshots from your router interface.
That iMac of yours is more than enough to get you going. If you plan to run multiple things on it it might be good idea to look for hypervisors like proxmox or ovirt, but basic qemu+libvirt -setup on pretty much any linux-installation will work just fine too.
For the 3rd part, your concerns are mostly about networking and setting up pihole/other servers on your local network will gain you knowledge on how to manage that as well. Also, you can set up nextcloud/immich/whatever locally at first, get familiar with them and then allow access from the internet either via bitwarden or other tunneling or directly over public network. Latter has obviously way bigger threat models than using VPN and accessing stuff that way, but gladly the networking side of things is somewhat it’s own beast from the servers so you can build everything local only at first and then figure out what’s the best approach for you with remote access.







Even if your router acts as an DNS proxy it shouldn’t overload any pihole installation unless you have a crapload of devices doing millions of queries per hour. My pihole manages all my devices (20-30 individual things) without any problems and even if I hit some rate limit it’s going to be a change to default configuration, not a immovable object on your way. Based on quick glance over that reddit thread a new router might be a good option, but that’s another easy-ish task to accomplish. I use mikrotik device and I’m pretty happy with it but there’s a ton of good options.
For hiring someone to coach you I can see quite a few of potential issues. People who claim to know what they’re talking about but don’t really have the knowledge, straight up scammers obviously, mismatch in personal chemistry which will make learning unnecessarily difficult or even impossible, some people just aren’t good at teaching even if they do know their stuff and so on. By all means, use your money however you like, but I personally strongly advice against it unless you can get some courses on (preferably local) reputable vendor. You can look for online courses too, cisco has a ton of courses on networking, redhat has plenty of linux courses and other big players have their own training and even certificates if you want to go that far.