I don't know of a universal tool for adding keyboard navigation to all the different GUI toolkits used on Linux. I wouldn't expect one to exist, since each toolkit implements its widgets differently, just as the Windows and MacOS GUIs implement theirs differently.

However, apps made with Qt tend to be good at keyboard navigation already, which is no surprise, since support for it is built in to the toolkit. The KDE Plasma desktop environment inherits this support, as do most of the apps made for it. I suggest trying it if you haven't already. (Hint: Many widgets will reveal their keyboard shortcuts when you hold down Alt, and a Settings: Configure Keyboard Shortcuts menu item is very common in KDE apps.)

Tangentially related: You might also want to look at tiling window managers. Some people love them.

This isn't a Linux thing exactly, but I know the QMK firmware for keyboards has a feature called "mouse keys" that let you control your mouse cursor and click and scroll and such exclusively via keyboard keys.

I suppose you need a program that scans the active window, highlights the clickable or scrollable areas and then the click hotkey.

Honestly, no idea...

Sounds cool though