I have a better idea.
Why not limit the upgrades to all weapons, armor, horses, saddles, etc. to three? The problem with upgrading any ranged/melee weapon more than three and they would either best a tombola weapon, GT, howdah or make the player too OP altogether if the enemy is slow on upgrading their weapons. I like the idea of having upgrades to make new chances of getting better equipment, but it is better to have a limit in order to prevent OP players from ruining the fun of others around them.
And for items that cannot be upgradable or if you want to upgrade single items (such as tombola weapons or event sets like Valentin's Set) you can still upgrade them, but they cost a lot more in nuggets (which will increase per upgrade) or in-game dollars for the upgrade to happen than they would be if you upgrade them normally (I call the normal method "three-tier" because it requires three items for the upgrade). And it makes the game a little more balanced so that all weapons, armor, horses, saddles, etc. can be upgraded. (For example in nugget terms, to upgrade the donkey one full level with just one donkey, it would require 30 nuggets to upgrade (or 1000 in-game cash for the first level). For a tombola weapon, for example, it would cost around 500 nuggets at the start for one level upgrade, 1000 for level 2, and 2000 for level 3 (even though it does not increase the attributes much)).