I'm doing a bunch of quests for the TG, and I want to know whether the Guild Chests are ok to store my loot that I want to keep.
Do these chests reset?

The chests in the sleeping quarters of the Thieves Guild don't reset, so they're safe to store stuff

Safe storage inside the Thieves guild. Now there's a funny thought. Lol.

Thank you.

And one of the beds is unowned, just for you.

"Welcome to the family!"

When you enter the Cistern from the Ragged Flagon, there's a bed on your right. That one's unowned, so feel free to use it. I tend to use the chest beside it & never had any issues of losing anything.