You can lvl your current decorations to an arbitrary lvl of diminishing return and then save up for the next deco. Lvl 10 for all and Lvl 20 for the ones you actually use and maybe more for the ones that would benefit you most, for starters. Or better, compare the decos via their cost and not their lvl.
That base cost reduction one is an odd one though. Only have it at lvl 25 right now and up till then, it es not really diminishing return. Rather increasing. May be heavily depending on the next few cost increases. Lvl 6 of the stock market costs 65k ...
Reply timer reduction ist obviously utterly useless in advancing the company, other than the prestige that skill gives.
The dl version registers more clicks. Since they do not have an idle mode, but rather an offline only bonus, they should just copy the concept I have seen in at least two other clicker games and provide the players with ingame clickers.