I found it extremely easy to find the "macos sequoia installer" to download from Apple's servers. Literally those search terms will do it; download -> install.

Admittedly it would be "easier" if Apple gave you the choice of which version to upgrade to in macOS, but that's generally not how software is provided.

> And it pisses me off that there's no easy way to upgrade to Sequoia (15).

Open the Mac App Store and search for Sequoia.

https://apps.apple.com/app/macos-sequoia/id6596773750

This doesn't work for me. I see it in the App Store but it says not available for my system and redirects me to the Tahoe update.

edit: worked after second attempt (via macappstores://apps.apple.com/app/macos-sequoia/id6596773750?mt=12 )