Hello Lutz,
I can at least answer your question about the cache: The Intel 660p is a QLC SSD (Quad Level Cell - i.e. 4 bits per cell). However, the speed of QLC is relatively low, so unused cells are converted to cache. Cells that are used as cache work as SLC (Single Level Cell - i.e. 1 bit per cell), which significantly increases the speed. In the case of the 660p, we are talking about 1800MB/s vs. 100MB/s. Logically, however, fewer and fewer unused cells are available as cache the fuller the SSD is. So if you write more data to the SSD than there is cache available, the speed will collapse at some point because the SSD first has to empty the cache again. Depending on how full your SSD is, this happens between 140GB (<25%) and 12GB (>75% full).
If you do not want to copy large files to the SSD regularly, the SLC cache should be sufficient.
You can find more information about this here:
https://www.legitreviews.com/intel-ssd-...
Many greetings