Hey,
I love questions like this. 
For context, I am currently editing and mixing a full-length (±75min) album of recorded physical instruments (-no midi use), -e.g. a drum-set, multiple guitars (-both electric and acoustic), bass, piano, organ, violin, vocals of all kinds (-large choir-like sections, singing, screaming), samples (-of different people talking, ‘nature’, etc.), –all of which change dramatically from song to song.
Now, to answer your question(s), it really depends on:
- What is the intended variety/diversity of the album in terms of layering, instrumentation, etc.?
- And are you writing songs first, then turning to Ardour to record them? -Or are you using Ardour as a ‘playground’ to create the foundation of songs directly into it?
In my specific case, I already had ±90% of the recorded material ready to go (-via another program from about a decade ago), and thus the track (and bus) structure was already more-or-less implied, –I just had to painstakingly create each routing structure in Ardour accordingly. So for me, really the only immediately-shareable thing (-from song to song I mean) was the drum-set track and bus organization, which leads me to…
In a nutshell, if the intended diversity throughout the album you’re working on is fairly limited, then yes, making and reusing a Session Template is one reasonable starting point. However, as the potential song/instrumentation variety increases, I’d suggest thinking more about what chunks (-of tracks/buses) could be shared instead, then taking advantage of Ardour’s built-in "Session → Import Mixer Strips…" feature (~thanks again, @x42).
For example, I created a project that I saved as “TGK Drumset Template 1” where I carefully refined exactly how I’d like my drum-set routing to be (-which I think has something like ±15 tracks/buses in it). From there, I then went into each relevant project and used the Import Mixer Strips… feature to copy just those routes into each.
Similarly, you could do the same for certain common guitar setups, vocal routing, etc…
Some degree of manual effort will be required for any desired, universal change, yes, and this just comes with ‘the territory’ of album work…
So again, say you’re mixing song A and you’ve added just a new, parallel compression bus to your drums that you’d like to also now use in songs B and C. As before, just open projects B and C and use Import Mixer Strips…, selecting just the new bus you’d like to add (-then proceed to update its direct connections and/or Aux Sends as required). Which leads me to pretty much the ‘the main takeaway’ here…
All in all, no matter what workflow you arrive at over the years, you will always be left with a hefty amount of manual labor all throughout the process. -And the more ‘interesting’/diverse the album you’re working-on gets, the less you will be able to ‘get around’ this fact.

Now, as for metadata, I honestly didn’t even know you could manage/manipulate metadata using Ardour until you mentioned it just now, haha. But my response is still the same: You might just want to postpone adding (or amending) metadata for later, and via another application designed specifically for adding/manipulating audio-file metadata. I’d imagine that batch-processing is possible, but I haven’t yet looked into this myself, so I don’t know what software to recommend, if any.
However, can’t you just manually change the metadata for one session, then use the “Session → Metadata… → Import Metadata…” functionality in every other project to get the new info.? (-Just discovered this too, of course, so I’m not entirely certain if it’d work for you.)
~Always fun to write/talk about album work. 
~Cheers, 
-J