What has changed in your brain after electroconvulsive "therapy" (ECT)?
There is actually quite some information on this. After a cursory Google Scholar search using the keywords &quot;long-term neuronal effect of ECT&quot; I found the following effects on the brain:

A week or two after the ECT, gray matter of depressed patients seemed to be increased in volume throughout the brain (Ousdal, 2020);
In rats, ECT resulted in behavioral improvements and neurogenesis in the hippocampus. Quite some of these new neurons survived up to 12 months (Vestergaard et al, 2017);
In mice, a single ECT triggered cortical snaptogenesis and a sustained neurogenesis, accompanied by behavioral improvement. Relapse occurred 40 days later. However, a 5-week continuation protocol following the initial ECS treatment led to a more persistent improvement of behavior and better survival rates of new neurons, but the authors did not investigate how long effects last after ECT was stopped fully (Jonckheere et al., 2018);

References
- Jonckheere et al., Brain Stim (2018); 11(6): 1336-47
- Ousdal et al., Biol Psych (2020); 87(5): 451-61
- Vestergaard et al., Hippocampus (2017); 27(1): 52-60