Thank you for trying out Trench and for sharing your concerns and ideas!
We believe that, in essence, what you suggest is already implemented and shown by default (please check the Game Preferences, by clicking the hamburguer menu in the top right corner of the screen).
Somehow, it may have happen that you had different preferences selected, but we suggest that you fiddle with them, as they may show you what you need. We like to give the player the possibility to customise the experience.
Although there aren't any arrows indicating the directions each unit may move, there are coloured overlays that show where each unit may move (and if any capture will happen).
For instance, if you check the first image in the "Advantages" section of the rules
here, you'll see that, by default, the selected unit is coloured blue, the moves without captures are green, and the moves with captures are red.
If the player selects a different friendly unit, no move is made with the previous unit, and the system updates the possible moves accordingly.
Note: at the moment, it is NOT possible to check which moves are possible for enemy units. This idea is noted as a suggestion
here. We don't plan on working on this in the following weeks, as we aren't sure how to implement this in fair way.
By default, the "Move Confirmation Prompt" is on. This shows a button at the top of the screen that you need to press to confirm that the move you made should be sent to the server. If you click a different unit or press "Cancel", the move won't happen.
We hope to have been clear and that your concerns were answered.
If not, or if you have any other suggestion, please share with is us so that we may improve this implementation even further!
All the best