The best way to check how makes/models compare is to look at the specs for each, and compare.
I have no clue re Elna vs. Janome, but that is where I would start!
First came the 6500.
The 6600 was an updated version, with some definite improvements over the 6500.
I had the 6600 and loved it.
7700 was about the same time period, more bells and whistles.
I looked at it, but there was nothing extra there, that I wanted.
While you speak of problems, there are a lot of people who loved and are still happy with the 7700.
The 8200 and 8900 were updates again, of those earlier machines.
Similar machines, just that the 8900 has more than the 8200.
If I were comparing, I am sure I would zone in on these two over the earlier models.
That being said .... there are more models since those ........
You really do need to do your research, and do a side- by-side comparison of any you are considering.
Of course, there is still the $-factor to bring into the mix!