There's a crowd calendar here: http://www15.plala.or.jp/gcap/disney/. It's in Japanese and kinda over-complicated, but you can run it through Google Translate.
If you can, I would switch to stay in the City first then do TDR on the weekdays. Weekends are EXTREMELY busy and frequently reach capacity. In any case, be sure to be at the gates at least an hour before park open - there will already be large lines, bring something to sit on. All that said...
I was there in the middle of January last year and crowds were not bad, but the parks were far from empty. By US park crowd standards, I would say it was like a 4-5 out of 10. It was cold, but certainly not extreme - not as cold as my home in Seattle that year. Practically tropical compared the...
If you want to avoid crowds, yet still have nice weather, I think the best time is the week after Golden Week (which is approximately the first week of May, so go second week of May). Usually lowest crowds of the year besides January. Weather is warm and the rainy season shouldn't have started...
By the way, the just updated Shanghai Disneyland app for iOS lets you purchase the Premier Access fastpasses directly from the app, either individually or the set. Tempting, but way too expensive.
The iOS app update to support digital fastpasses is out NOW! No idea if it's actually working in the park yet. It also lets you buy "Premier Access" fastpasses directly in the app.
Looks exactly like California's MaxPass, only NO EXTRA CHARGE. Supposed to be available Fall 2017. This should take care of the fastpass scalping I've been hearing about.
http://disney-magical-kingdom-blog.blogspot.hk/2017/09/shanghai-disney-resort-digital-disneys.html?m=1
OK, I did find the info: https://www.shanghaidisneyresort.com/en/guest-services/disney-premier-access/ . It's really buried deep in the site and not promoted or linked-to in any way. Also looks like you can only buy it on-site.
I just checked the official SDL web site and there is no mention or information on paid fastpasses. And there is no option to add them when purchasing tickets. Are they only available on-site or was it just a test? https://www.shanghaidisneyresort.com/en/guest-services/fastpass/