I would argue that preferring to reuse the same space of the SSD instead of using new space as you propose (cow, et al.) is the better approach. This way you don't need to use trim that much and the SSD will anyway write it into a different place.

It should be noted that while an SSD can't write before erasing it will actually not even attempt that. The SSD maintains a mapping from LBA to physical flash chip and offset and will write to a completely different place. It will then mark the old location as unused and erase it when it can spare some time and power.