Should also check to make sure you don’t have any utilities buried in the path you want to trench. Many localities will have a number you can call and they will tell the various utility providers to flag any buried infrastructure they may have in the area.

100m is probably fine but you are at the limit of the spec. If you don’t want to deal with re-running the line if you do end up have transmission issues, fibre is better.

Probably worth double checking local regulations but most don’t care about low power lines like Ethernet. Definitely put it in a conduit though, to both protect it and make any future modifications easier. Should also cover it with a layer of different medium, such as sand, so you know when you are near it if you do need to dig it up again.