No, you're not crazy. I am seriously thinking about the same thing, as the Asian parks are my last 3 to visit. Coincidentally I posted a similar question on my Facebook a few hours ago. And yes, I think it can be done. It will take time to plan and put money aside, but it's not impossible.
Step 1: How much time do I need?
I am thinking I will need 2 - 3 weeks. Preferably, go to the parks on week days and use the weekend days to travel and sightseeing Tokyo, Hong Kong and Shanghai.
TDL: 3-4 days
HDL: 2-3 days
SDL: 2-3 days
Step 2: When to go?
As we have no data yet on how the crowds in Shanghai will look, I have started with checking when the crowds in Tokyo are the lowest as Tokyo is a lot busier than Hong Kong. So either January or the week before Golden Week. Crowds are lower in January, but also winter in Japan and January is not the best time for me to take off time from work.
Step 3: How to get there?
There are many low cost carriers within Asia, so looking into booking long haul flights separately from 'domestic' flights. Try to figure out what to costs are for the 6 different orders of visiting you have (THS, TSH, HTS, HST, STH, SHT).
Step 4: How much money do I need?
Investigate hotel costs, compare on site vs. off site. Investigate food/souvenir costs per park/city. Investigate sightseeing option + costs.
Wherever you go in the world, you can do it super expensive or on a budget.
No, you are still not crazy.