Why do people hold TGs from one year to another?
Either because they have an excess due to recipes not requiring what they generate or because they don't care to make some year's transmute. However, if the latter is the case, then either the transmute isn't that good or someone else will want to make it - average number of made. If the latter here, then you sell TGs to those people. If the former, then that's a development problem.
Even given how small the community is and how few relics/legendaries actually need to be made, there are still so many elements to behavior that have to make some general assumptions. A general assumption I would make (then design/develop to it) is that the same number of relics/legendaries will be made each year.
As for treasure affecting the TG balance, while that's rather important for DS/MH, making some complicated adjustments for other stuff. I suppose you could use an uplift factor of some percentage of stuff. Obviously, Golden Fleece aren't directly buyable, so would have to do some calculations for that, anyway. I'm just not seeing someone doing a lot of runs with a small buy. May look like such people exist to TDA, but I don't buy direct anymore and I put way too much money into tokens every year. Should be a rather high correlation to how many tickets buy and token amount bought. Note, not how many runs, as someone could do one run but buy 10 tickets.
Side stuff does throw off any sort of calculations. Mystic Orb recipe seemed less planned than Stalker recipes, for instance. As fluffy as trying to align generation to cost may be, how much components cost just doesn't relate in a usable way to transmute pricing.