Here are some update....
I completed the transition on the other PC. It took multiple days until all tracks had been analysed the additional data. I didn't do them all at once as I had multiple crashes..
After all was done I made a backup from the converted DB.
Track browsing is very slow - nearly unuseable.
I checked to connect my iPhone XR to rekordbox 6. It took very long as it transferred all Playlist data already to the phone before I even transferred 1 track.
I synced a playlist and it worked fine. Useability of the app seem to be very nice. Connection to my XDJ-1000 was don without any problem. This app is much better compared to the previous version.
As the PC I was doing all this testing is an old one, I decided to install rekordbox 6 on my main PC, to verify how it works on a more recent computer.
Instead of converting the database again on the other machine, I restored the backup from the test PC and relocated ther files.
This went pretty well and did took that long. All together maybe 30 min...
On my recent PC (Intel i5, 8GB Ram) rekordbox 6 is much more responsive compared to the other machine. Scrolling is fine, but it takes a few seconds to see all album art or waveform data incl. cue points for oall tracks shown on the page.
Main difference of the test PC and the main PC is the use of SSD drives, which seem to be have an important role in here. The test PC got the database and the music on a traditionnal HDD. The main PC got music and DB on an SSD drive. This seem to help the responsiveness of rekordbox a lot.
I know have rekordbox 5 (incl. rekordbox DJ & DVS) and rekordbox 6 (without any subscription) on one machine. I have both DB's stored on separate drives (both are SSD drives).
For now I will maintain both libraries and keep them in sync and continue to explore rekordbox 6, especially the iPhone app which to me is a big step forward. As my time as a club DJ is over (did it mayn years the 80ies and the 90ies), I am not planing to subscribe to any paid plan. But my 2 XDJ-1000 mk1 might be due for an update, depending what is been released in the near future. Let's see how this connects to rekordbox 6 - maybe as HW unlock device...
My DJM-900NXS2 is not due for an upgrade and I hope it will last for a long time.
Btw.
Because I saw many upset comments from users mentioning the competition, because of the move Pioneer did towards to a subscription model. I am following the D****N community and their new players looking great but the software is far behind rekordbox functions included in the free plan today. And I don't see that changing soon.
But this is my 2 cents and I do not so much rely on the features in the paid plans as I invested in more standalone players. I totaly understand all the noise we see right now about the change. The future will show in which direction is goes and if the market acepts it or not.
After all the frutstration in the migration process on my test PC, I am pretty much happy how it looks now on my main PC.
But as Pulse already recommended - I got backups from nearly all steps I took :-)
My DB is too much important...
PS: One very big recommendation would be an easy way to keep the rekorbox 5 and rekordbox 6 DB in sync. I understand that because of the technical changes made to the DB, it is no longer downward compatible, but an easy way to keep them in sync - at least until rekordbox 5 may become obsolete would be very welcome.