An awful lot of time is five years?
Even if you want to min max I still think that forming the HRE as Venice would be a better maximization even if you miss out on five years of doing something, and you don't because you can use your croatian vassal to expand in the mean time.
I mean sure some people might not want to join the HRE and that's fine but if your intent is to do the best with Venice I can't see how being in the HRE would not be that great. Unless republics are much better than I thought.
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You were talking about becoming emperor, and forming the HRE. That is not simply 5 years of investment. That is centuries of investment. and certain choices of idea sets become required, that you could ignore if you chose to seek your power and fortune elsewhere through trade and naval might. The HRE is a landser's game. Your galley stack is not going to stop france, poland or even denmark. Your lack of diprep and authority bonuses is going to require near 100 IA for each reform.
Now if all you really care to do is join the HRE to keep austria off your back, by all means that 5 years could be well spent. But it'll bite you in the ass later, especially after Landfriede.