I have used and owned Apple products since 1992.

Absolutely agreed about the gatekeeping — just as example, you cannot even update your MacOS without being online unless you create a full USB Installer.

For your particular situation, presuming your hardware supports it, you can use MrMacintosh guides [0] to downgrade your OS from Tahoe — I recommend the latest Sequoia installer — all officially downloaded via Apple links.

[0] <http://www.mrmacintosh.com>

I further recommend blocking *.apple.com & *.icloud.com (and many more hosts) using a PiHole (so simple, so necessary).