This is exactly the kind of content I love to see from the community, and exactly the kind of use I would have for old mainboards. You have a rather ambitious list of goals, but it is definitely not hardware limited! 
I do not mean to deter you from your goal, but I wonder if you have considered virtualization as an option to reach all of your needs on the software side? For example, it seems like there are a couple of different concerns which might best be handled separately:
- Routing/networking/firewall
- NAS for backup
- Potential media sharing possibilities (i.e. the mention of streaming with Kodi, Iād add Jellyfin/Plex)
Separating out these concerns, one possibility that arises is the use of a barebones, virtualization-focused OS like Proxmox to create separate VMs or containers for these functions. For example, it is not uncommon to virtualize an extremely feature-complete routing OS like Opn/pfSense to function as the router/firewall (pfSense on Proxmox, OpnSense on Proxmox + with HA, you could also use OpenWRT as well). Then you could use separate containers for NAS/filesharing functions as well as streaming. The VM focused on routing would come with advantages, since these battle-tested, routing-focused OSes have many features baked-in. Iām not as familiar with the OpnSense side of things, but all of these are available on my openWRT router:
- QoS
- DMZ
- Guest networks
- Fail2ban
- Various VPN server/client protocols
- A pretty web portal for configuration, as well as SSH access with command line
If youāre using 11th gen, you likely donāt have the CPU cores to create separate VMs for all of these functions. Youād probably want a VM for the most security-related function (firewall) but the NAS/streaming-related functions could probably work well using the LXC containers supported by Proxmox.
Of course, youāre probably looking to make this more āappliance-likeā than this, but I thought this could be an interesting option with some advantages. Best of luck on your project!