Happy to help if I can. Another thing you could try with QOP is adding a small amount of a custard (ready made or concentrate) to see if that tempers the acidity of the fruit. I do suspect Cookie may do the trick though rather than introduce another different flavour to QOP.
I've often added a bit of custard to things that were a bit 'lacking' in something.
In trying four new concentrates from chefs which just about finished off their line - any other lines there which you think are great and so more than just single flavors. Aka proper concentrates like the chefs line? Still waiting for yaeliq's juice to show
You could take a look at the Chefs Choice (10-20%) range or the Rebranded (1-5%). Just looking through them will give you notions of what would be good mixed with something else, but lots are fine by themselves.
They have ranges that I've never tried before, and generally speaking, I prefer the ones that use low percentages of flavouring unless it's something I particularly like.
Having said that, some, such as Mom & Pops sound good (They have full descriptions on their website) so may be an idea to check for reviews on this forum.
One thing I have found useful is to find a few suitable good basic ones: doughnut, cheesecake, etc., and add various flavours to them: Doughnut can have any fruit added plus chocolate, cream & custard, etc.
Cheesecake likewise for fruits, and with the addition of chocolate, caramel, butterscotch, etc. If you make up say 100ml of the base liquid and then make smaller bottles of various flavours that would suit it, you can customise and vary them to suit, and perhaps find a few firm favourites.