Bramwell, given that you like the Suehiro Rika so much, the first thing you have to do is ask yourself what you think a 6K will do better than the Rika -- which is 5K if you work the mud a little.
You'll get some subtle differences from stone to stone, but those differences will be more about the stones themselves than the differences between 5K and 6K.
"You may find softer but I doubt your going to get an abrasive feel from any 6k. As far as softer I think your on the right track with the Chosera, the 6k Naniwa Super Stone might be even softer."
Naniwa doesn't make an SS 6K. The SS 5K is a pretty good stone, and is a lot of polish for the grit rating, and may very well be the stone the OP is after -- similar in a lot of ways to the Suehiro Rika. Not quite as creamy or packed with friendly feedback, but you don't have to jack around with breaking down the mud to make the SS act like a 5K. Naniwa doesn't make a Chosera 6K either. The Chosera 5K is faster than the SS, but slightly harder and with more bite. Most sane people would go for the Chosera. Personally, I prefer the softer SS. You (Bramwell) prefer the Rika.
Arashiyama/Takenoko is a very fast polishing stone with enough speed and reach to get all the way down to a King 1.2K, and polishes way above its nominal grit rating; but it's hard, slick, and doesn't provide a lot of feedback.
Lots of people like the Shapton GS 6K. If you've ever used a GS, you'll know whether you will or won't like the 6K. They all feel about the same. Very, frikkin' hard; heap much plenty bite.
The Gesshin 6K is probably as close to what you want as any nominal 6K I've tried. Good stone, fast, friendly, communicative, but expensive. And tell me again why you want a 6K?
As long as you're polishing, you might as well go to 8K - 10K worth of slick and shiny. Tons of great stones in that range.
Naniwa SS 8K is very soft; in fact ridiculously so, and will gouge very easily if you're not careful. Otherwise, could be perfect. I had one which I liked a great deal until it started to crumble and began to gouge on sight -- partly because it got used up, and partly because its first owner treated it like crap. For pure feel, the 8K Naniwa Pure White leaps to mind, friendly price (considering the quality) too.
But let's get the horse back in front of the cart and talk about getting to 8K. The Suehiro Rika is not only an good final stone in its own right, but an excellent way to get to just about any 8K, including the Pure White. Moral of the story: Suehiro Rika.
Another great feeling and reasonably priced possibility is the Kitayama; but the Kitayama won't reach very far down, and really wants something almost as fine as its nominal 8K rating as a lead in. In fact, "as fine" or "even finer," might be better. I think a Rika should just barely reach the Kitayama, but haven't tried it myself. What I can say empirically is that a Shapton Pro 5K lead-in was OK, but just OK.
Probably getting way out of the practicality and budget ranges, but... The Pure White chased by the Kitayama is a magic edge. Obviously I haven't tried everything but the Pure White/Kityama is one of the two best edges I've ever been able to get off of stones. The other is the 8K Gesshin.