If you are decided on B6’s for the front, then why not go for the B6 option on the rear? At least you will have a known starting point and can then chop & change as you wish. Given you have a 29 year old car, anything will be an improvement over what is on there now if still original fitment.
Waylander has fitted B6 rears only but does have relatively new standard fronts- just a mere 12 years old, and is very happy with the level of performance from the current set up. At some point, he will let me hack his front struts up so I can do the B6 conversion on them but for now, what he has works well for him. Which I guess, is kind of the point; suspension is subjective and about feel - what feels good for one person may be a bone shaker for someone else or a wallowy old pig, not that I’m suggesting that Waylander’s 968 is either!! Just trying to illustrate the example.
As you don’t intend doing trackdays and want to retain originality and the rear ARB, the B6 option ticks all the boxes.
If you want to consider helper springs, then aren't these part of a coilover solution which goes against your original requirements if I have read your post correctly?
There is lots of information about the various options that members have tried on the forum, but it’s a subject that has been done to death and unfortunately it is a case of using the search function with perhaps different key words to filter down the results to find the feedback that you might be looking for.
Before you spend any hard earned cash on any new boingy bits, get the car properly set up by someone that knows what they are doing and can do so for how you drive the car. Get their feedback on whether or not it would benefit from new kit - it may be that just new bushes all round is what’s required to give a new lease of life.
I wish I could offer some insight on the performance of the B6 rears but all I have done is fit them and the car remains on the lift awaiting further attention
Didn’t see Waylander had responded whilst I was writing this...