Hi Grace,
Unfortunately, all resins are going to yellow over time. Resins with UV inhibitors and stabilizers help extend the time before yellowing occurs. However, things such as mixing off ratio (even though it still can cure without tackiness), putting over an improperly cured lacquer, excessive heat, and or constant exposure to sunlight can speed up the yellowing process. How long the epoxy will last in its clear state is very difficult to predict.
UV resin isn’t a good choice for a project like this because it’s going to be hard to get the resin to fully cure because of the thickness of your cast.
For a project, like this, I would use the Resin Obsession deep pour epoxy resin. You can pour it up to two inches thick at once and it cures very hard. You can buy it in our store here: https://shop.resinobsession.com/collections/resin/resin-obsession-deep-pour-resin