Hi Richard,
You do not need any subscription for the boxed Premium Deluxe edition. That license is perpetual and is tied to the Microsoft account you redeemed the 25-character code with. What you are seeing is almost always the Xbox app or Store showing the Game Pass listing because Windows is signed into a different account than the one that owns your entitlement.
The fix is simply to use the correct Microsoft account and launch from your Library, not from the Game Pass page. You can confirm ownership by opening Microsoft Store on the PC, selecting your profile, then My Library. Your purchase should appear there and you can Install or Open from that view. If it does not show, check the Order history for the account you redeemed in back in 2020.
Do this exactly. Sign out of both Microsoft Store and the Xbox app on the PC, then sign back in with the same email you used when you entered the 25-character code in 2020. Open Microsoft Store again, go to My Library, and start Microsoft Flight Simulator Premium Deluxe from there. Avoid the green Play with Game Pass button. If the boxed build asks you to insert a disc, that is a license check specific to the retail DVDs. Insert Disc 1 and it will authenticate, as long as you are signed into the right account. If you ever see an 'Insert disc' or 'license cannot be authenticated' message even without discs, it usually means an authentication hiccup and signing out and back in to Store and Xbox resolves it.
Try the above and tell me what you see in My Library. If your Library does not show Microsoft Flight Simulator on the account you are using, we will track down which account holds the license and get you back in.