GameCube hacks are becoming really popular. There's the newly released FlippyDrive which allows booting games from a sd card without soldering or removing the disk drive. I'm a huge Mario Kart: Double Dash! fan and there's many fan made tracks and fan made roms which includes more tracks and kart.
Are folks generally using CRTs or is it common to use a newer upscaling approach?
The first american GameCube (DOL-001) has a port that can be used to output in 720i. With that port you can use a device like the popular Carby or any device that uses the GCVideo software. You could make your own.
For upscaling there's Prism HD, a more expensive device
https://insurrectionindustries.com/product/carby-v2/ https://github.com/ikorb/gcvideo https://retro-bit.com/prism-hd/