Great, it seems like you already have found the optimal settings for the job. These are my thoughts for inspiration:
2 TByte data with app. 1GByte per file, that is app. 2000 files, so the default of 8 checkers is fine. If you primarily had many folders with small files, then more checkers might be better.
Your 15 Mbit/s connection has a theoretical max. upload around 1.5 MByte/s (using 8 bit/byte + 25% overhead). My rule of thumb is to divide by 10.
You are seeing 1.6 Mbyte/s, so 1 transfer is enough to saturate your connection. This is sufficient because you primarily have large files (e.g. videos), you would need more transfers if you had many small files (e.g. photos or office documents) or a faster connection. Repeating myself for new readers: Using more than 4 transfers may trigger OneDrive throttling that will be slower than 4 transfers.
I fully agree, in your situation it is a good idea to reduce transfers to 1 (or maybe 2).
One thing I do not understand is your
--max-transfer=1G when your files are between 0.8 and 3 GByte - I guess it is just for your initial testing.