@johnjohn123 here is one approach
https://youtu.be/2jeG-1uScaU
This will also use a custom Form but a User has to click a Button on that Form to be redirected to the Standalone app.
When you customize a form you get a SharePoint Integration object. On that object (it will be visible in the tree view on the left) you have several properties like OnEdit, OnView, OnNew. Here you could change the existing Formular to Launch(urltostandaloneapp)... You could also pass a query String so your Standalone app would recognize if the User clicked New item or edit item.
Is the current SharePoint Custom Form Implementation Perfect? Probably not. But as @Pstork1 mentioned it is for free. Will it be enhanced in the future... Very likely yes. Will it ever have the same.functionalities like custom coded Apps, Nintex forms, Spfx Web parts, etc. Nobody knows.
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