Idea
Add a personal upgrades section in the upgrades shop, and move equipment there changing how combat upgrades work entirely.
How it works
At the start of the game you can only buy an iron sword.
This would be called ‘sword upgrade’.
Once you buy the iron sword, the diamond sword takes its place, still under the ‘sword upgrade’ name. And since there are no upgrades beyond diamond sword, the button will turn gray.
The diamond swords price will be reduced to 100 coins, 200 coins in total for it.
The same could be done for armor even if there was only the one upgrade, however I think it would be good to have diamond armor aswell for 200 coins. Vote for that here.
This would allow new items to be added without clogging the shop up. Of course, this personal upgrades tab could also include more upgrades in the future.
The main suggestion here is for personal upgrades.
If you dislike the diamond armor bit specifically, still vote for this post but not the post i linked.