I'm also headed to Disney alone in January. Advice:
1. Book a few TS meals. I'd suggest no more than one per day. The weather in January can be iffy - and if it's cold - the restaurants will fill up with folks looking to escape the cold. If you decide not to keep the ADR, just be sure to cancel the night before.
2. Prepare for a range in temps - could be 50 degrees in the morning - and upper 70's by mid-afternoon.
3. Book some headliner FP's - you can change them later. Solo or with 7others - there are some rides that are just a pain to get on due to lines. While you can control WHEN you try to ride and if you want to wait as a solo, you can't fix the 45 minutes wait problem. For my upcoming trip - I haven't picked all three per day yet - but I do have the key ones in place. I might change them up as I go - but at least I know I have one trip on my favorites that tend to have lines.
4. I do spend more time on my phone on solo trips - so if you normally risk having the battery run out during the day - yup - a back-up charger is a good idea.
5. Use the Photopass photographers and in addition to the photos they take on their camera - give them yours. They do this. It's what they do. This way - you will get some pictures that aren't just selfies!
6. Use your instincts about starting up conversations with others - especially in lines. I do this some and it definitely is nice to occasionally talk to someone - even though I do these trips to be an introvert.
7. Since you haven't been there in three years - be sure to spend enough time on their website before you go so you know park hours, new attractions, etc. You can do a TON solo in a day - much more than when traveling with a group - so I find I get so much done so quickly - that I underestimate how much I can do in a day. I personally find it hard to pre-plan TOO MUCH on a solo day. And the great part of being solo - if you have that crashing headache that just won't go away - you can go back to the resort and not ruin anyone else's day!
8. If you like character meet n greets - don't be afraid to interact with the character yourself. You waited in line - you can have a minute or two with the character - just like those parties of 8 that take FOREVER getting every combination of the family group in the picture. I haven't meet Joy and Sadness yet - so I'm looking forward to that!