A simple way to get around Renoun. Go into SH5's main folder/CFG folder/Basic folder and edit all renoun to 0. But make sure you back up the original Basic cfg file first. Then you can choose when you want an upgrade or not, and not have to worry about renoun.
I look at it this way. Uboat commanders never had to pay for upgrades, weapon improvements, or ammo and torpedos. So why should we have to. I try to keep any improvements close to the time they were available in real life back then. And sometimes I say the hell with it and use whatever I want, no matter what the date is where you are in the sim.