What else can I try for frequent GPU timeout crashes?

Try running without the dedicated GPU for a while. The 7800X3D has integrated graphics, so you can remove the card and connect the display to the motherboard. Make sure the PC is powered off and unplugged before removing anything.

If the crashes stop, that narrows the search toward the card, its power connections or its interaction with the rest of the system. It doesn't prove the card itself is bad. If they continue, the dedicated GPU becomes less likely to be the whole problem.

Check the GPU power connections at both ends. Shut down, unplug the PC, and reseat the connectors at the card and, if accessible, at the modular PSU. Make sure they're fully seated.

The PSU is also a possible culprit, but a timeout alone won't tell you that. Without a spare, a shop could test with a known-good supply before you spend money replacing one.

The 0x141 dump tells you the graphics engine timed out, not necessarily why. The AMD driver files in the report are useful clues, but they don't by themselves prove a driver bug or a faulty card. Crash Defender may be recording the failure rather than causing it.

Keep the watchdog dumps and a short record of when the crashes happen for the shop. Since they happen at idle too, include that rather than describing it as only a gaming problem.

You could check the Hardware Accelerated GPU Scheduling setting in Windows and test the other setting if you haven't already. It's a relatively quick software test, though a change in behavior wouldn't necessarily identify the underlying fault.

Windows itself is another possibility, although clean system file checks make an obvious corruption issue less apparent. If the hardware tests don't narrow it down, a shop can help assess whether a fresh Windows installation is worth testing. Back up your files first.