It is simple. Why do you think it’s not simple? Just use external .ase file. That’s all. You can save anytime. You’ll have to create another sprite with Tilemap anyway to handle usage of external tileset in either format.
I’ll repeat — in EITHER format.
I just wrote about how to do it in details. So what to be afraid of? Do you afraid of when somebody describe details of simple things? What is the difference between saving to .ase and saving to .asetiles (example extension). What is the difference between opening .ase and opening .asetiiles? The only difference is that you cannot edit .ase tiles directly, but have to ask editor to open another tab for it and edit there. But editor will open it automatically in this case, so what to worry about? For internal edits you have internal tilesets.
I think we can have both. External .asetiles tilesets (just internal tilesets saved to external file) and external .ase tilesets (edited manually). The first is editable during work with Tilemap layer, the second is editable by auto-opening new tab and have more power of combining layers, including sub-tilemaps for metatiles. Everybody is happy.
At least there should be way to export .ase into .asetiles format. But it will only slow down the workflow.
The biggest danger if developers will make external .ase files for tilemaps, but will allow to reference only to internal tilesets there. Now this will be really useless.