Is it possible to read erased flash memory?
The problem with secure erase is that the device has a translation layer in it. On a disk, writing to cylinder X sector Y will always overwrite the same area. On an SSD, the device firmware maintains a list of blank blocks and writes to the next one available, maintaining a table mapping logical addresses to actual flash blocks. There is usually a bit more flash than the stated capacity as spares, so a complete write may not touch every block. Erasing blocks is slow, so they are not erased immediately, and maybe not erased until space is short.

Drive firmware may offer three possible solutions to this. One is TRIM: tell the drive which blocks do not have filesystem data in, and let it preemptively erase them. One is a specific "secure erase" command, which should actually erase all the blocks (but takes a long time). And one is transparent block-level encryption, where asking the drive to discard the key instantly loses all the data. However, you're hostage to how well this has been implemented and there may be bugs.

I'm not aware of techniques for recovering data from flash cells that have actually been erased. (In fact it may lose bits spontaneously, so error correction is built in).