This is in user voice as well but till the time we get solution from product team, you can follow this workaround so that only Powerapps form is visible to users and they can just utilize submit button you have designed.
User Voice link:
- Make your Powerapps List form ready.
- Create a Page on your Sharepoint online site
Go to --> settings -->Site Contents --> Site Pages --> New --> Wiki Page
3. Get the Power Apps form unique app id.(which you can locate inside the address bar of the browser after clicking customize the form)
4. Now insert that APP-id in below script - iframe
<iframe width="1024px" height="768px" src="https://web.powerapps.com/webplayer/iframeapp?hideNavBar=true&source=iframe&screenColor=rgba(104,101,171,1)&appId=/providers/Microsoft.PowerApps/apps/c9hjkhkj0-23ca-485f-8245-19ghjiijea13
5. Now go back to the page that you created, edit page and click edit source.
6. Insert the code and save.
You will now just get the Powerapps form without save and Cancel button.
I hope if you are in same situation where you do not want users to use default save and cancel buttons, this approach will surely assist you.
Note: Do not forget to change the Powerapps form to open by default in New mode rather than edit.(else you will see the filled item when user opens it)
Thanks for reading