Games

menus include pause menus, talent trees, inventories, all that kind of stuf

Right, which is why I specifically said there's an exception for menu-heavy games like 4x and grand strategy. If we're mostly talking about launch and pause menus (which was my intent), that's a small scope of work, as in weeks, not months.

You can absolutely build that in a few days, and then redo it later once UX has decided what needs to go there. It's pretty similar game to game, so build it properly once to be data driven, and then tweak the UX and options a bit for each game. Optimization is generally done pretty late in the dev cycle, so those options don't need to exist until later in development anyway, and that's like half the work.

The important thing is to have your UX team iterate on it before your devs get involved, so it's ready. And have them build it out while optimizing things for release. Your menu systems don't need a ton of testing relative to the actual mechanics and gameplay.