First reaction is that I don't like it.
18 month loan will take him through his 23rd birthday, by which point his age value will drop pretty significantly. If you doubt the validity of age value, look at the difference between Dallas getting $20M for an 18-year-old Pepi vs us getting bids for less than $15M for 23-year-old Taty.
Maybe he really shines for Rangers and gets transfer bids from other clubs. I still think we'd have to wait until the end of the loan in order to sell him, so he's basically locked at Rangers for 18 months even if he plays his socks off and gets his valuation higher. Money now is always better than money later, and 18 months is a pretty long time. If he flops at Rangers, he'll again have to just flop there for 18 months before coming back here at a much lower value.
Then there's quality of play vs playing time. In comparing the Bundesliga to SPL, is it better for Sands to play consistently in a lower-quality SPL or start as a backup and (hopefully) work his way into the starting XI for a higher-quality Bundesliga team? Sands has said he wants to play at a "top level in Europe", and the SPL is definitely not a top level in Europe.
Sands in the Bundesliga makes too much sense to me. But I'm not a GM, so what do I know? Whatever happens, I wish the best for him.