1. We usually have upgrade coupons when the version for the next year comes out or starts pre-orders. If you upgrade later in 2021 you would likely pay the full price unless you wait for a Steam sale.
2. The SAT is a collection of tools: it includes the Substance Designer Python API but can be ran as a standalone, it also includes command line tools for the bakers.
3. You can request access to the SAT if you have your license linked to a Substance account.
4. Linking your Steam account will give you updates for 1 year starting on your purchase date, rather than for a calendar year. It will also of course allow you to not have to launch Steam to use the tools.
5. You won't lose your Steam license, you will just have both a stand alone and a Steam license.