With loot duels i have received multiple antique chests, I have unlocked all the rares for this rotation, If I were to keep them in my inventory, would subsequent promotions reward me different antiques? Or should I just be redeeming them for the stars, tokens, etc.
This depends on whether Jagex uses different antique chests per rotation (in which case saving up doesn't help) or keeps altering the rewards of the chests each rotation without programming a different antique chest that looks the same from a player's perspective but is actually a different item (with new rewards).
You could test it out and keep your chests unopened for now until the rewards rotate, but you have to give up inventory space for that, which only you can decide whether it's worth it. For some it's prolly no big deal, but if you use presets you have to say goodbye to them potentially for many months until Jagex update the rewards (no one knows when). I wouldn't do it myself I can tell.
Feel free to take a look at my suggestions:
200m xp prestige