Re IS Sunstone Trilliant... I'll try one more different way of making this case.
Choose Your Own Adventure
Suppose the legendary trilliant at -10 exists. You're designing a combat room in which it makes sense for a monster to deal Fire damage to the whole party.
How much damage do you have it deal?
A) 10 Fire, because a priori that seems like a reasonable number in context.
When you turn to page 42, people with the trilliant literally laugh it off; everyone else takes reasonable damage. Player opinions about the room vary widely as a result.
B
) 20 Fire, because we don't want people with trilliants laughing it off entirely, especially since it seems like half the players these days are running around with trilliants.
When you turn to page 69, people with trilliant are happy they wore it and say "ow", but everyone else says "OUCH holy $#!+ this dungeon is way overtuned, TD is such a pay to win game, etc etc"
C) 10 irresistible/Eldritch "fire", because the other two choices are both lousy now that the world is full of OP trilliants.
When you turn to page 394, absolutely everybody is unhappy. People with trilliants feel they wasted money/resources building a trilliant, because what's the point if it never does anything; people without trilliants are sad that their Flameguard Greaves don't help prevent a little bit of it even though they guessed right about the dungeon having fire.
My point is not to recommend which of A,B,C is least bad. My point is that choices A and B are both much *improved* if the trilliant is -5 per element instead of -10, because in any given room there's less discrepancy and more balance between the experiences of different parties based purely on one expensive token. Everyone with the trilliant will still be happy they wore one. The legendary is still worth making for those with the means to do so, because it protects against more different things so you don't have to guess which one will be most relevant (same reason UR Greaves of Absorption are better than the rare Greaves), plus extra saves. Nobody has to suffer the misery of Choice C.
Note that the proposed overall DR cap of 20 doesn't actually help because it's barking up the wrong tree. It's not that having e.g. -10 Fire under any circumstances is bad; a budget player can look through
tokendb.com/subtracts-damage-from/fire/
and get to -10 Fire with four Rare tokens, or higher (with diminishing returns) if willing to devote additional slots to the effort, but that's fine because it's a substantial tradeoff for the opportunity cost of all the other things you could have put in all those slots. By the same logic, I have no problem with someone building to -30 Fire if they have to use up a bunch of slots to do it; work hard and accomplish great things! The problem is getting -10 from a single token.
Please consider making the relic and legendary trilliants -5.
I'm fine with including darkrift, and I also don't mind if they overlap elements with SRoEC (SRoEC is already broken and there's no fixing it now, that ship sailed long ago, though at least it takes up a valuable slot. I just want to avoid having this new token break things that
aren't broken yet)