If your PC has a decent firewall (e.g. most GNU/Linux distributions) set to block incoming connection requests by default, it is safe but honestly pointless.

The extra speed you seem to be seeing isn’t going to impact your normal Internet usage, only your speed tests.

Note: Many 10G nics aren’t capable of 2.5G and will downgrade to simple gigabit with devices (including switches) that are 2.5G.

The point of 10G nics is faster data transfers that most home networks don’t really need or noticeably benefit from, although during something like a full backup/restore to an NAS on your home network you might benefit.

Yes, it will be an issue. Eventually.