Over the last few days, I experimented with moving my Twitter archive to my personal #Mastodon account @luca@social.luca.run and finally succeeded.
I had to modify the Mastodon source to allow backdated posts and prevent it from spamming other instances with old posts. Because image descriptions aren't included in the Twitter export, I had to request them from the API. There I got full text for truncated Retweets as well.
Incomplete notes:
https://github.com/lucahammer/fediporter