Licences on a USB eLicenser are available on whatever computer the eLicenser is plugged in to - you do not need to activate and deactivate each computer.
You can have multiple versions of Cubase on a computer. If your existing computer meets the requirements for Cubase 10, you can leave Cubase 7.5 installed and run Cubase 10 as well. If your computer meets the requirements for a later version of Cubase (Pro) than 7.5 but not Cubase Pro 10, you can install that version of Cubase and use it with your Cubase 10 licence until you upgrade your computer.
If you activate your upgrade now, your Cubase 7.5 licence will be replaced with a Cubase Pro 10 licence, though this should work with all versions of Cubase (Pro) - Cubase licences are supposed to work with older and/or lower versions of Cubase, though you might need to update eLicenser Control Center and run its maintenance procedure on the machine with the older version of Cubase, also there might be problems with very old versions of Cubase (others know far more than I do about the intricacies of eLicenser).
As you point out, there is no link between Steinberg online store accounts (run by Asknet) and My Steinberg. New licences only appear in My Steinberg when you activate them on an eLicenser and that eLicenser is registered to your My Steinberg account.
The only reason to delay activating your upgrade is if you want to wait until the grace period for Cubase 10.5 starts (or, to be even more certain, Cubase 10.5 is released). If you do this, you should get a free upgrade to Cubase 10.5. If you delay activation, make very sure you have bought and received the correct upgrade and keep the activation code safe - it will be almost impossible to sort out in several months time once the sale has ended.
As -steve- has said, the current Steinberg system is that you simply download Steinberg Download Assistant, which allows you to download Cubase Pro 10 with no need to enter an activation code or even to have a licence. The download is, of course, of no use unless you have a trial or full licence for Cubase.