It could be or maybe the SSD has its own on-firmware TRIM schedule, but all major OS's execute a TRIM on a time based schedule no longer than every 10-15 minutes.

Perhaps, but this is unrelated. The magnetic charges may still be there, but if the reference to the content is deleted, how is the filesystem meant to know what file is there? This seems really suspicious to me.

This can happen when TRIM is disabled

Here's a study published last year I read that goes through this exact thing

In consideration of results obtained from the experiments, it concluded that the behavior of Wear Leveling in different SSD manufacturers having the same storage capacities does not match. It varies based on the number of files, types of files, and sizes. The recovery of files from different SSD manufacturers showed different results. In all SSDs, not a single trace of any file found in disk format scenario(s). Whereas, some of the data recovered in the delete case and from only one drive. It clearly showed different behavior of data recoveries in format and delete cases. The obvious finding from this study is that the time interval of image acquisitions played a significant role, and the longer time interval supports few chances of data recovery because the TRIM and Garbage Collection process effects clearing residual data from the drives

Non PDF link

Edit: corrected links

I doubt that the firmware is doing an overwrite of TRIMmed data. Rather, I expect it's marking it as having been TRIMmed, and so can report that it's zeroed to higher layers. If a higher layer queries the firmware for its content, sure, they might get zeroes returned. But if you can modify the firmware or otherwise bypass it, you may be able to get at the underlying media.

TRIM is garbage collection and is a part of the wear leveling system. The whole point of TRIM is to have the SSD only hold the charge it needs too for still in use (i.e. not deleted) data. It's the charge that damages blocks over time, so to extend lifespans it clears everything not needed. It's not overwriting data for security or anything per se, but rather just a result of its longevity processes

Now, I'm sure there are cheap no name SSD controllers out there with ineffective TRIM operations that just lie about the operation, but any controller worth its salt is gonna have proper TRIM.

There is also the "bad block" issue, where storage media can take blocks -- which may contain readable data -- out of use, so that higher layers cannot access them. That applies to rotational drives and it looks like SSDs do the same thing. Again, might require bypassing or modifying the firmware to get direct access. But there can be data leaked there.

Part of that process is to move the data to another block and release the charge to prevent further damage, it's possible the block is damaged in such a way that it won't even release the charge, but if that's the case it's incredibly unlikely to be readable.

I also wouldn't be terribly surprised if there is lingering information even after zeros are written to an SSD that might be recoverable if you could directly access the media, though I'm not familiar with the situation there. That is the case for rotational drives -- the drive platter itself is "analog", doesn't just store a discrete string of ones and zeroes at the physical level. I once knew a cryptographer who was working on quantifying that leakage for rotational drives.

Yea it's possible, but now you're in the needing x-ray machines, powerful microscopes, full clean room labs and people with extensive, specific skill sets which means $$$$$$$$$$$$$$$$$$$$ or in other words, state level budgets range. 99.99999% of people will be fine