Immediately I say, I weakly understand the API and which is the difference between them. From August 1, in Google Play, new applications must be with API 28.
Idea has the ability to put API 29.
But I understand that this is not the final version, and also Beta 5?
Will there be something else with the API 28 or is it final?
Better to start a new application on 28 or 29?
Answer 1, Authority 100%
I would recommend to use 28. Not all AndroidX libraries could drive up to 29, for example, there may be bugs and other joys of life, yet beta. And there are no advantages for an ordinary developer. When a new android and playmarket comes out, says that it does not want to eat anything below 29 – move to 29, this process takes place without problems in the overwhelming majority of cases
Answer 2, Authority 50%
The most reasonable solution is to set the maximum stable version. So you have everything to work on all devices.
Put the beta version you only need if you want to figure out what it will be necessary to rule there, so that when it becomes stable – do not study it in a hurry.
Without verification, if you work on your new version of the API, the application is dangerous. Often breaking the broken changes that are not activated if you do not have the latest version of the API.
Answer 3, Authority 50%
API 28 This is the current (last at the moment) Stable version of Android. API 29, this is the next version that should go out.
Better to start a new application on 28 or 29?
It is better to first decide for whom and for which devices or goals you write the application, and at the same time estimate how much time to develop.
If it is an educational application, then without a difference. You can even start immediately on the API 29 to look at the novelties (IMHO).
If it is some kind of working application, then definitely API 28, because This version is stable, and in Bethe there can still be a lot of unpleasant surprises (bugs, etc.). Plus, it is probably necessary to wait so far for API 29 will be compatible library updates, etc.
p.s. In addition, I doubt something that you will allow you to lay out the application based on the raw API (again my IMHO).