Which kobo did you use? If it’s one of the color ones, it’s going to be much more grey than B&W e-readers due to the extra color filter layer. All the color e-readers have this drawback and rely on the front light to improve contrast.
If you don’t need color, just go with the latest gen of a black and white reader, which should be using Carta 1300 I believe, with the software you’d like. Could also go with an older generation if you want to save a few bucks, such as carta 1200, all you would sacrifice is a bit of contrast and some responsiveness.
One point for above ground is that it is far easier to know when it’s damaged to the point of being unsafe for the general public and much simpler and quicker to repair. For underground, you don’t know that until there is a failure that causes outages or someone/something gets hurt.
While I have seen numerous downed power lines, I have not know anything actual hurt by them. On the other hand, I have known multiple dogs who’ve died stepping on top of electrified access points while out for walks. While this is purely anecdotal, it’s not black and white either.
Other underground utilities have more obvious failure signs to the public (smells, flooding, water damage etc) and generally have minimal short term consequences while electrical faults tend to go unnoticed until a significant failure event (i.e. power goes out or something gets killed). Our town has hundreds of reported natural gas leaks, that is take years to fix while pole repairs tend to happen within an hour of being reported with police standing by until the crew shows up.
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.