If I buy the PS & LR CC package ($9.99/mo.), will I lose my LR Classic?

What is the EXACT Version number of the Lightroom version you are now using?

To find that out go to the Help menu item in Lr and select either System Info or About Lightroom.

You can NOT buy Lr Classic. you Subscribe to the Photography plan and that comes with LrC and (and this is a BIG AND) Photoshop. There is NO other way to Subscribe to Lr Classic.

So either you are totally confused on whatever version of Lr you are currently using, as it CAN NOT BE Lr Classic unless you already have a Photography Plan subscription or you do have that subscription plan and you don't realize you can install Photoshop as it comes with that plan.

"I'm looking at buying PS again (many years since I've had it - and failed to use 99% of it), and I see the $9.99/mo. package for PS and LR CC. But ... I'm already a longtime user of LR Classic, and hated LR CC when i first tried it in 2017, and when I realised I could just buy Classic, I was SO RELIEVED."

Unfortunately, you can't simply "just buy" Classic. And at no time in the past could you "just buy" Classic. The only way to obtain Lightroom Classic was (and still is) via a subscription to the Photography Plan which includes other software, like Photoshop. So it's really not clear what you are talking about.

If you currently have paid for a subscription to Lightroom Classic, then you also have a license to use Photoshop. It's all part of the same plan. There's no need to buy anything. Just open the Adobe Creative Cloud app and download Photoshop.

(Side note: there is no longer any software known as CC)

The 20GB plan includes both LrC and Lr. Together with PS you have the best of both worlds to try. Originals from Lr desktop or mobile go straight to the cloud. Or you can sync smart previews from LrC  collections and they don’t count towards your 20GB storage but are still available on all devices.

LrC = Lightroom Classic

Lr = Lightroom Cloud eco system (Win/Mac desktop, mobile & web apps)