All these images are done in MSpaint, just a rough concept so calm down. I'm not a PSpro. Focus at the functionality idea presented here, the quality of life features.
And sorry for the upload quality, if the images seem frizzled, first time posting images. Open them in a new window for full size.

Most important one first. The "bank".
Having to talk to each retainer (even though I only have 2) is so unoptimised.
The open/close back and forth spam is insane.

Make it all show up at once.
I managed to fit it easy.

Everything should be explained in the image.
You click on the summoning bell, you get whatever retainer you prefer to greet you with a "hello" which you do not need to click away. The summoning bell opens the inventory instantly.

You see all the 175 item slots in one view. Item type icons on the bottom, toggle them to highlight / take / deposit all items with that category tag.
Click click, all done. In and out. Items or stacks that have been modified get a slow glowing highlight frame that fades away over a few seconds, so you see what happened, or what went where. Optional.

You can browse your retainers by picking the tabs on top. Easy. No more back and forth talking or exiting the previous menu chains.

On the very top you have a toggle for the venture page.
I'm too lazy to edit it, but just imagine a list of retainers on the top too. So you don't need to go back, but can send them all to a mission/venture from the same page.
All the missions, availability, restrictions, conditions, rewards, timers in one place.

Last image shows the third window for gearing retainers.
Clicking on a item slot shows you the best/suggested 3-4 items in your armory/inventory. Just check if he has the best stuff available and you're done. No more oogling at random items in your armory to see if it's better or not, or if he can use it or not etc.