Hi,
I have over 20 years experience in programming with more than 15 years in web development and quite some years with mobile apps.
As far as I understood you already have some existing API and now you want some Ionic app for it developed. The main part is some google map with markers + OAuth
I have worked quite often with ionic, as it its the framework of my choice for all apps I develop for customers. So I am quite familiar with it.
Can you please send me some more infomation about what you need in concrete and the mockups, when they are ready.
I do the "agile development" approach - meaning, that through the entire process I update you with new versions, as soon as it has some major changes / new functions. You can than check the progress and we can talk about how you like it and what has to be changed.
I would start with a MockUp - setting up the user interface for the app, every button, ... will be there but with dummy data and clicking buttons will not do anything (in most cases).
Then I will make this app alive by adding functionality.
I will be happy to hear from you soon.
Best regards,
Fabian