Ext4 is gonna be the fastest for general daily use. The extra utility of copy on write filesystems is stuff like efficient snapshot backups, optional compression at rest for your files, and really really fast copy operations.

AMD's drivers are fully FOSS and built into Mesa, whereas Nvidia's are closed source, so yes, AMD GPUs are more reliable and less of a hassle on Linux. However, Mint makes dealing with Nvidia drivers easy, so it's honestly your call there.

The AMD cards have better performance with the in-tree driver than Nvidia does, so it is just a little bit easier with an AMD card. But it's really not that hard with Nvidia, so you do you.

Ext4 is perfectly fine for most uses, although some distros will default to btrfs. Whatever the default are in the distro you use are going to work fine

1. Filesystem doesn't matter hugely but as the other user said, ext4 will be the fastest anyway (possibly xfs, not sure how ext4 and xfs compare). CoW filesystems like btrfs are slower, though most people don't notice a significant difference. People use CoW filesystems for other features like self-healing ability and backups.

2. I would strongly recommend getting an AMD card. As the other user says, AMD's drivers are fully FOSS and work well with Linux. Nvidia has a bad reputation with Linux and especially Wayland, though these days it's mostly usable, but IME is still prone to breakage upon updates. IME AMD GPUs "just work".