I'm always hesitant to throw out dungeon gear if there's any chance I'll use it, because RNG can make it a pain to get back.

Something you can do to free up space is temporarily hire a retainer - they keep your items permanently even when you're not actively paying for access to them. So subscribe to hire one, get charged for that one month*, and immediately turn off that subscription again. Dump your "low-priority" items in their inventory, make use of them while you have them, and then leave them to long-term store those items. Re-hire then for further dumping or retrieval as required.

* assuming your subscription is month-to-month. From memory hiring a retainer automatically charges you to the end of your current subscription period, so be careful if you have a six-month subscription or something.