Love the game! Quick recommendation: the whetstone feels super OP to me. I would consider either limiting to once per turn, or maybe only lasting for a turn, or reducing the effect.
It is OP but not super, there are crazier stuff out there, its just too easy to use so you saw it easy. It becomes less and less usefull in infinite floors because it doesnt give insane burst like mage build can neither can it do enough exponentiality as it is limited by your energy. I can only see it go brrr if you fill your inventory with berserk rings realy haha.
No.
To expand on Toranet's original comment, the Whetstone is quite frankly meh as an item. Each time I do get one in my inventory, it is usually there to take space. I almost never use it due to the marginal damage bonus it provides.
Additionally, there are better items to use over the whetstone. The ring of doom provides +2 damage points to weapons near it when used, but given that it is a magic item that costs 1 mana, I can use it a lot to ready myself up while keeping my energy in tact. The poison whetstone, in spite of being usable only once per turn, deals way more damage to the enemy because that gives the weapons +1 poison which stacks and not only that, but the poison takes a while to go away, so the player is constantly dealing damage to the enemy while the enemy can not do anything about it.
There is one occurence in the game where the item does get pretty broken, it is whenever the player has a shiv/hatchet/cleaver build because the whetstone affects every weapon in its row and column, so you can get +30 damage worth out of one whestone if you have 10 shivs.