SSD’s dont work like old HD’s depending on the generation of tech it might be storing multiple values per cell which means when you “filled” the SSD you put a charge into every single storage cell on the drive.
Garbage collection and TRIM will slowly over time clear out all the cells flagged as deleted but if one bit is still valid in a cell that was holding 3-4 other bits it cant overwrite that, or relocate it.
That means that your files/videos and such stay fragmented and may never get put back together sequentially or in a way that the controller can optimize again for speed.
The only fix, may be running a factory wipe from the Drive MFG’s tool set, that should fully blank each cell and let you re-install and make it feel fresh again.
Be warned though, you have already done a full drive write once at least, this would be another. You can expect some dead cells and while there is over provisioning that should provide replacements you could see a loss in usable space sooner than later.