I've been ranting about trading since Civ VI was released that could have some simple fixes. Most seem like intentional or lacking game design vs true exploits, but still can have a huge impact on the game.
For one, things that work per turn should only be allowed to be traded per turn. For example, I shouldn't be able to trade luxuries for one time gold payments. This let's you trade lux's and then immediately declare a surprise war after you just got all of their gold while restoring the luxury back to you instead of being gone for 30 turns. Alternately, I can offer a ton of GPT for a relic, etc. and then declare a surprise war to stop the payments after I have the relic.
Deity can be easier than lower levels in some ways due to the INSANE amount of gold other Civs will trade you for things. Setting a low percentage limit for the amount of gold civs will trade at higher levels would fix this. Something like 100% on King and below, then decreasing to E-50, I-25, D-10%. Even at 10% you could still get a ton of GPT on Deity.