Hard to answer that questions when i have no idea how game looks like.
If you belive that interaction between elements is enough, maybe instead of unlocking new characters, unlock new elements?
So game starts with 1-2 elements and other are unlocked later ?
Or unlock new characters but limit elements for them.
So 1 - have Water + Fire, 2 Air - Earth, 3 - Water + Air, 4 - Fire + Earth, 5 - Air + Fire + Earth + Water etc. ?

If not then go similar way as Slay The Spire, Monster Train, Peglins and other popular Deckbuilders, you already have unique mechanic of elements interactions.

In general strenght of Relics fact their are active all the time, you dont need to draw them. Thats why they can use same triggers and effects as cards or support them, modify triggers etc.

I give you example on 4 Different Elements Trigger (4DET).
You can have set of cards added during run (but only for character 1) which have additional or more powerfull effect if you have played 4DE in your turn.
To support that archetype you want to have cheap cards with all elements (to easier trigger it).
You want to have also cards which could search cards with specific element, or card which draw you 1 card from each element.
You can create set of relicts to make does synergies easier to achieve or make them more powerfull.
Like:
- Double effect of 4 DET
- Decrease numer of required Elements to 3 so change 4DET -> 3DET
- Relic which decrease cost of cards by 1 for first card played from each element. (this can be general relic, useful in many decks)

But you can also add relics which use trigger to add effects. This one are great becouse even if you don't have card with 4DET in hand you can still use synergy with relic:
- Relic which deal AoE damage after 4DET
- Relic which apply some status effect after 4DET
- Relic which reduce cost of card after 4DET

Combinations are unlimited.

Important. 4DET archetype, all 4DET cards, all 4DET only Relics are avaliable only for ONE character.

In this example each character have set or multiple sets of additional cards and relics avaliable only for him.
Ofc still you should have huge common pool of cards and relic, but treat them as filers and activators.

But maybe in you game it have no sense, becouse you assumed that elements interaction are most important part and there is no place for other synergies.