Not sure why you need a new router for PiHole. If your machines all point to the Pihole for DNS, it works. Router has almost nothing to do with what provides DNS, other than maybe having it’s DHCP config include the Pihole for DNS.
Even then, you can setup the Pihole to be both DHCP and DNS (which helps for local name resolution anyway), and then just turn off DHCP in your router.
As I understand it, Tailscale and Nginx fulfill the same requirements. I lean toward TS myself, I like how administration works, and how it’s a virtual network instead of an in-bound VPN. This means devices just see each other on this network, regardless of the physical network to which they’re connected. This makes it easy to use the same local-network tools you normally use. For example, you can use just one sync tool, rather than one inside the LAN, and one that can span the internet. You can map shares right across a virtual network as if it were a LAN. TS also enables you to access devices that can’t run TS, such as printers, routers, access points, etc, by enabling its Subnet Router.
Tailscale also has a couple features (Funnel and Share) which enable you to (respectively), provide internet access to specific resources for anyone, or enable foreign Tailscale networks to access specific resources.
I see Proxmox and TrueNAS as essentially the same kind of thing - they’re both Hypervisors (virtualizatiin hosts) with True adding NAS capability. So I can’t think of a use-case for running one on the other (TrueNAS has some docs around virtualizing it, I assume the use-case is for a test lab, I wouldn’t think running TN, or any NAS, virtualized is an optimal choice, but hey, what do I know? ).
While I haven’t explored both deeply, I lean toward TrueNAS, but that’s because I need a NAS solution and a hypervisor, and I’ve seen similar solutions spec’d many times for businesses - I’ve seen it work well. Plus TrueNAS as a company seems to know what they’re doing, they have a strong commercial arm with an array of hardware options. This tells me they are very invested in making True work well, and they do a lot of testing to ensure it works, at least on their hardware. Having multiple hardware products requires both an extensive test group and support organization.
Proxmox seems equivalent, except they do just the software part, as far as I’ve seen.
Two similar products for different, but similar/overlapping use-cases.
Best advice I have is to make a list of Functional Requirements, abstract/high-level needs, such as “need external access to network for management”. Don’t think about specific solutions, just make the list of requirements. Then map those Functional requirements to System requirements. This is often a one-to-many mapping, as it often takes multiple System requirements to address a single functional requirement.
For example, that “external access” requirement could map out to a VPN system requirement, but also to an access control requirement like SSO, and then also to user management definitions.
You don’t have to be that detailed, but it’s good to at least have the Functional-to-System mapping so you always know why you did something.