@GabiW
Thank you for sharing this precious experience! Could you elaborate a bit more how galleries, videos and attachments could be moved into fields with SQL, the rest is clear.
Yes, I adopted your approach using as few 3rd-party plugins as possible long ago and placed all my thrust on K2. It gave me piece for long years, but now the headache is ... I got few sites that are alive from J1 with many hundreds K2 items translated by FaLang (there was no native multilingual J at that time), each with a gallery, video, attachments, extra-fields and many sub-templates.

@krmr
On my website there is a small guide in German and in English translation, maybe that will help you and others.
jelabit de / migration-k2
Best regards,
Gabi

[justify]Krakatoa, complete migration tool from K2 to Joomla Contents, coming

I am pleased to present the tool for the complete migration of data and files from the JW K2 content manager to native Joomla, which will allow it to be updated to version 4 and 5, without losing any functionality. I have tried all the options that exist (all paid) to carry out this task, but none of them do it completely or they are not done correctly, such as not respecting the nesting of categories, not processing attached documents, image galleries, videos, comments, fields or tags. [/justify]
Krakatoa does! 


etereastudio . com / blog / migracion-completa-desde-k2-a-contenidos-de-joomla . html

Just a heads up for those of us who using the unofficial K2forJ4 version on Github.
1. It has recently (2 weeks ago) been updated, which is encouraging to see.
2. Acymailing's J4 versions no longer support K2 integration for newsletter content.
I asked them about this and they wrote the following, which makes total sense:
"I'm sorry but for the moment I don't think that we will make an integration with an unofficial extension because when they will release the version for version 4 it will be very different."
I have several J4 sites running the unofficial version - so far, so good.
-Alan

I also confirm the happy news - after the update K2forJ4 works like a charm on the last J4. The only trouble is Sig PRO but that is irreparable so far.

Hello @JoomlaWorks. Should we wait for the update of the K2 component? Or Should we look for alternatives? We really love your component. It will be very difficult without him.

Thanks for the news!
We still hope there will be a K2 official J4/5 version soon, but at least K2forJ4 is something.

Whenn I try to install K2 on Joomla 4 or 5 I get an Database Error. An I can not create items

Are you using the unofficial K2forJ4 or the official k2 (which is not ready for J4 or J5)?

As a lot of people here we stumbled on the same problem that K2 does not have a version for J4 & 5.
We updated our JSW CRM app, already almost 10 years available in the Joomla.org extension directory, so we could replace K2 in our projects and client websites we manage. 
It's not a clone of K2 it is different but with all the features you need and more..
Visit this page about JSW CRM where we published what it can do ...
jswebdesign.nl/j4-components