Good for you, William. I admire your adventurous and enterprising spirit. I like what you've done in the Althyria suite - very wistful and mysterious with lots of dramatic twists and turns, and I love the opening with the simple yet very atmospheric arpeggiated harp and celeste accompaniment for the wistful leitmotif.
Re-intoning a piece like Althyria will involve quite a bit of work, and I'd suggest trying Pythagorean intonation (PI) to start with and see if you like the results. Eventually it may even turn out that this piece is best left in ET for the sake of the characteristic bluesy minor third in the delicate principal leitmotif.
Just intonation (JI) in practice is unfortunately an EXTREMELY awkward mule to use as an overall intonation schema, and really a JI piece must be composed in JI from scratch. I spent several years wrestling with JI until I concluded that the theorists were full of s**t. And indeed it turns out that in actual practice JI has never ever been widely used - it's little more than a theorists' pipe dream of using pure, natural intonation for everything. For an excellent historical perspective on JI, see:-
Meantone temperament (MT) was, before ET replaced it, the schema of choice for centuries in European keyboard instruments, especially church organs, since it does away with JI's practical awkwardness yet sounds sort of, kind of, similar-ish to JI. Moreover, unlike JI, MT can be scored normally as if it was standard PI.
The vast majority of 'exotic' tunings are either attempts to overcome the limited modulation range of fixed 12-note tuning in PI or MT, or are non-European and thus require special cultural understanding to use properly. These days I have little or no interest in any of them. I've tried out a few classical Indian scales but gave up on them because modulation as we know it doesn't exist in classical Indian modal music - traditional modal music in Europe also never involved modulation. So I've stayed focused on the two most historically significant intonation schemas used in Europe - PI and MT.
So let's look at how you might get started retuning your Althyria to PI. Unfortunately and weirdly, in the collection of Scala-format .scl microtuning files provided with VIPro, PI appears to be missing. Not to worry, we can easily make them in VIPro. I’ll provide a list of PI microtunings in cents for all 35 notes in standard western music nomenclature, at the bottom of this post. There is a Scala editor app intended for creating and editing microtuning files in .tun and .scl format, but it's ancient and the most horribly nerdy app I've ever encountered - I loathe and detest the damn thing and certainly don't recommend it.
Hope you don't mind, I've made a rough sketch of the first several bars of Althyria in Logic to try out VIPro's microtuning. I'm very new to the VSL world and have had VIPro only for a few weeks. Plus I'm currently struggling with my "freecasting" subsystem in an older and pretty flaky version of Logic, since in a recent update, Apple broke some stuff that's essential to me. (Thinking of migrating to Cubase Pro.)
I'll add the next bit to this post asap - hopefully within a couple of hours.