Man the older titles hated screen window messing around but I got a little workaround with dgvoodoo.
Did the collectibles with sun's batch file cuz the crashes were sooo many if I shifted tabs it'd take another day to clear em, but after I was done, I still needed the table for other stuff and ofc THAT windowed mode didn't cut it, sharing my settings of dgvoodoo which gives borderless feels but doesn't crash nearly as much, very rarely and prolly when shifting/transfering from missions and that too when the cheats are enabled, normally even disabled it'd crash the game cuz the table wasn't the problem, the alt-tabing was.
General Tab
DirectX Tab
GeneralExt Tab: This tab unlocks by right clicking the logo and selecting "Show all sections...."
Didn't change anything else, and it also makes the game wrap in dx11, which unlocks more things idk about, I only do that for ReShade, it works just fine, use it on almost all dx9 titles, much better handling tbh. Should work the same with AC1, 2, Rev as well.
Ofc you can mess around with the settings, idk how they'd react but other than the screen modes, changing other stuff shouldn't mess with the workaround.