I have thoughts on migration, if I may add to your discusssion;
Firefish has post migration, and it is terrible as it spams every post to the timeline as it imports them to the new instance and reposts them. It is not a good implementation from that aspect.
I personally think they should not be migrated anyway. My feeling is that your posts being able to be downloaded and saved is enough, as realistically most will never be looked at again. Most of my accounts auto delete posts after a couple of months, so I don't have cruft hanging around behind me.
The exception is my blog which I spun up on WordPress and it federates as well. Blog posts stay, as that is why they are there.