OK, but a bronze +9 ring has 400 attack while a silver +10 earring has 200 attack. Why should I upgrade the earring except of course if everyone already has a fully upgraded ring?

Edit: Ah never mind. I am guessing that at some point I will be replacing the bronze rings with silver ones anyway, so upgrading them is a waste.

At first, you'll just put all mid-grade equipment as a priority over all low-grade equipment. Doesn't matter much which ones you use, because you probably have other things you need to use your gold on. Then when you build up a big enough supply of mid-grade equips you can start prioritizing within them.

The items, in order of power addition, are: ring, bracelet, earrings, necklace. The gaps between each type are big, so it's seldom appropriate to deviate unless you want to bulk up all attack or all defense for some reason.

Personally, the only worn equips I ever upgrade are +10s. Eventually all of the ones you use will be +10 so you don't want to waste gold and XP in lesser equips for what is really a marginal gain early on. I do 'bank' XP in lower equips to save space by upgrading them once with one equip of the same type to get them to lvl 8-11. This also lets me upgrade a new +10 to lvl 30 in one shot and at a much lower total gold cost than half-stepping it with lvl 1 equips. Gold will become less scarce as you play, but the equipment inventory space is still a huge reason to keep on doing this.

Always look for the most efficient way to level equipment. Doing so will save a lot of time, equipment and coins. I have created an image showing the most efficient way to max a silver equipment [ring]. PM me on the discord if you want more help, im almost always on there, rarely here.

I only upgrade High Tier equipment of Rarity 9+
Its expensive and takes a long time to do, so anything less than lvl 9 item is not worth the trouble.
Also I used to have all my girls equipped but with the way the game is set up, and the items are not "taken out" of your communal inventory, I had to unequip all non-active knights.

Normally in any game that has inventory you have "active" and "Passive" - passive is the one where you carry stuff, active is the one where you equip stuff. When equipping item, it gets moved from INVENTORY into EQUIPMENT (any RPG, Crafting and Survival game out there). This game is an exception (a stupid one at that) that ANY equipped item is still counted as part of the inventory, thus it still counts towards the maximum number of items you can have.
With that being said - equip only your active girls, otherwise you will soon run out of space, and with no space you cant roll for new - maybe better Rarity - equipment.

The *only* things I upgrade are the lvl 10 silver ones and the personal earrings. Why upgrade a lvl 9 when it's going to be replaced with a lvl 10 eventually? So I rather upgrade a lvl 10 necklace than a lvl 9 ring. And I have far from enough spare jewelry to upgrade all my lvl 10 jewelry, so it's not hard to "only level what I won't replace".

Upgrading a necklace with spare necklaces and upgrading rings with spare rings boosts them a bit better, but just boosting a single piece with whatever 10 spare pieces you have is more cost efficient. I tend to go for the median of that. If most of my spare pieces are rings, I use it to upgrade a ring. If most of my spare pieces are wristbands, I use them to upgrade a wristband.

I've made a a guide based on my experiences with equipment leveling and after researching different ways for each equipment. I only focused on what i thought was important (RIP earring). Take it with a grain of salt or treat it as a trusty page to follow.
And this is my current ring collection

Ok, thank you for your input everyone.

As a final question, is there a noticable XP loss for 'banking' items? If not it doesn't really matter how you upgrade stuff as you never lose XP. You can then simply sacrifice your previousy upgraded crap item whenever you get +10 items.