Native Android Interval Timer (MVP)

I am evaluating the development of a simple Android MVP for a gym-focused timer app.
The core concept is intentionally very simple:
- the user selects a time interval, for example 60 / 90 / 120 seconds;
- chooses the alert type;
- presses Start;
- the app automatically plays a beep and/or vibration at every interval, without requiring any further interaction.
The goal is to allow the user to keep the phone in their pocket and maintain a consistent training pace with minimal distraction.
Key technical prerequisite before full development:
the project should first pass a functional test proving that the recurring alert works reliably with the screen locked/off and the app running in background for an extended period, also considering Android battery optimization and background execution limitations.
For the MVP, the main requirements are:
- Android app;
- minimal and intuitive interface;
- preset intervals + custom interval;
- Start / Pause / Stop;
- countdown to the next alert;
- beep, vibration, or both;
- optional pre-alert before the main signal;
- reliable operation with screen off and in background;
- user preference saving;
- basic analytics to monitor usage, sessions and retention;
- architecture ready for future Premium features;
- multilingual structure, initially possibly only English and/or Italian.
No user accounts, cloud sync or complex features are required at this stage.
The objective is to launch a reliable MVP quickly and validate the market.
It is also essential that the source code, repository, project files, Google Play account, Firebase account and any servers/services used remain under my ownership, with the developer added only as a collaborator where needed.
I would therefore like to receive an estimate covering:
- MVP development cost;
- estimated timeline;
- proposed technology stack;
- any recurring costs;
- post-launch support;
- pricing/model for future app updates and feature development.
Confidentiality and ownership note:
all information, functional concepts and project materials shared must be treated as confidential and may not be disclosed, reused, copied, resold or used to develop a competing or substantially similar product without written authorization.
A specific NDA and software rights assignment agreement may be signed before development begins.

The attached image helps illustrate the interface concept (even though this app is different). Launch app -> set duration (60s, 90s, 120s) -> set alarm type -> start.