Any tips or tricks to speed up my test writing activity or my migration process?

Here is one piece of advice. Since your tests will mostly consist of automation tasks (if this condition is not met, you might want to reconsider using Appium), make interactions reusable! 

Do not write the same sub-scenarios twice in your tests, make a diagram of what your scenarios are and split them into sub-activities; you will get a graph where some nodes are reachable from more than one node. So make those tasks parametric and call them in your tests! This will make your test writing experience better even when you need to migrate from existing tests (hopefully you already did this activity for your existing suites).