I dislike how people underestimate non-session hours in sieges..
non-session hours in sieges are extremely important, let me tell you why..
when there is active battle session during siege, you are focused on movecraft resistance, AKA offensive and defensive movecraft, you are focused on getting kills, wasting enemy resources and economing your own, and most importantly.. banner, capping it, getting people off it, reversals. taking out enemy cannons, taking out enemy howitzers who do massive damage to the fort, doing non ending 90 minutes of pvp.
what do you think all of this will do to the defenders and attackers, i'll tell you..
attackers -
they lose potions which is extremely limited to them, they have to refill it in off-session AND during session, they might run out of any material that is needed including, energy, shells, armor, duct tape, and they most suffer from lack of anvils due to high infantry number.
they need to reprint their movecraft and also organize another push for next session.
meanwhile defenders repair their fort, replace their cannons, and also regain their resources which is quite easier for them, depending on defender of course.

during off-session attackers/defenders gain chance to put pressure on their opposition, meaning they can keep them in active combat and non-safe situation, which means camping rooftops, camping claims not allowing side to supply the base with certain movecraft, before update u could even drill in off-session but even after it got patched, off-session is yet still very important siege part.

the amount of pressure u can and are putting in off-session has high value.
your suggestion mostly says that off-session is an ease. but in reality, its sometimes siege win condition.

KEEP INVENTORY SHOULD BE REMOVED

Alternatively, disallow bombing of the fort during sessions and have keep inventory on during siege breaks only, so it is an actual break and not just session without the banner capping factor.

Disregard my above comment I didn't realize no bombing during session break was already implemented*

Either way, I do agree that keep inv should be removed during session, it is extremely unsatisfying it is to spend a ridiculous amount of time fighting at a siege for virtually 0 gain for anyone besides the owner of the fort/leaders of the besieging nation. The best current loot drop is food unironically.

Ijust, you can still do that while session is off, I'm just saying that people CAN take breaks now OR take fights without heavily risking EVERYTHING, off session is an integral part, and fighting is still very important, but the session is a much more important part that should be with higher stakes than off session, which is why I propose to only keepinv off during non sessions, you may still fight, u may still stop them with movecraft repairs, stop them from resupplying, but at the cost of less risk and less reward from getting sets

it gives both sides breathing room which incase doesn't let a 1 sided war 1 sided anymore

Here is something I find interesting: I keep hearing that sets collected by PvPers are locked away in large collections, never to be seen again. A continuous transfer of sets from weaker players to PvPers doesn't seem healthy for the game, especially when nothing is apparently being done with them.