You won't miss too much on TLG itself as the water level is boringly low right now. But TLG can be warm and sunny due to its microclimate, and with low season tourist levels, it's peaceful and nice there right now, but probably boring, since the river isn't even 1/1000 as impressive during wet season. The back road out of TLG to Haba and Zhongdian is an off the beaten track route, so you would miss that, but nothing more days in the Nujiang area won't make up for.

The upper Nujiang valley and even further north then Bingzhongluo is better than the more eastern valleys. (You cannot cross the Tibet border as a foreigner) The interaction with locals is better there as well then the more commercialised TLG, Zhongdian (Shangri-La) and Deqin areas.
Note as well that in January TLG, but especially Deqin and Shangri-La are very cold. The Nujiang is much lower and therefore slightly warmer.

Recent posts are useful for me in planning 10 - 12 months from now. The original OP is planning for June 2018.

The January references was for Sunrise2012 who wants to come then.

Just got back from TLG. It was nice and peaceful there. A stay at Tina's guesthouse for a family of 3 was only

200rmb. A clear day made for spectacular light on the mountains from the setting sun, then the full moon lit it up in silver. It was our first time there, so I don't know what a full river looks like, nevertheless the fact that the area was quiet made it well worth it.

Too cold for all but the hardiest of Chinese tourists. Actually this time of year, just before Chinese New Year is a good time to travel. People are saving up for the BIG holiday. Also the two months after CNY are quieter times to travel.