Decided to just give up on the achievement, but was wondering if there's an easy way to remove all of the items from my wardrobe?

All you need to do is go into wardrobe and just click on every item you want to take out then just click the option checked to inventory and keep doing that until everything you want out is out of wardrobe

It's easiest to just "Remove All" on your outfit and then wear the items you don't want, then Check All + Checked to Inventory like said, or you can go into the "Wardrobe" tab on the left and check off all the items in that list instead, then push the >>> button and Move to Inventory.

But the removed items don't disappear from the wardrobe if you do it the first way so you might want to refresh every so often if you have a lot of wearables to go through. And just a note, when I worked on those achievements I sorted everything I didn't want into a specific drawer so that I wouldn't have to look at them while I was using my wardrobe and it made it easier to remove since I just went to the Wardrobe page and checked full lists of items from that drawer to remove them. So if you're giving up because of the clutter, that definitely helps.