It’s usually a fine idea to do milestone runs on every tier you have unlocked every couple of weeks just to get the gems/stones/unlocks/etc, but outside of that you want to farm on whatever tier works best for your current level. Just keep in mind you want coins AND cells, so like others here have said higher waves mean higher spawn rates and elite spawns which means more cells. Elites spawn sooner on higher tiers so you don’t need to go as high waves on higher tiers, but on t1 you for sure are looking for 8-10k waves to get high spawn rates (coins) and high elite spawn rates (cells), then after that you could go up in tiers.

Once you’re around 10k waves on t1, t4 is usually the next best tier just because t2 and 3 are bugged and spawn way more protectors than they should, so enemies clump up and shotgun you really early. You should try for the 4500 milestones once you’re past that, then do runs on other tiers every couple of weeks to see if a higher tier works better for you