Hi Kaslink,
First, thanks for supporting the brand with your Blast/Jag, glad you like them!
Let´s look at the widths of your current 2 boards- Blast 115 66cm/Jag 135 82cm. So that is quite a big gap, give the Blast shapes are fairly narrow vs classic free riders. But as you say, can still be used with bigger sails if the wind is not super light.
I would actually tend towards the Jag, it is quite a bit livelier purely due to the size than the Jag 135, have sailed both again recently in Tarifa in different conditions and also did all the R&D on them with our team - really nice board with a good bit extra top speed, works great with the 7/7.8 and even 8.6. Width is 75cm, so I think it fits perfectly between your 2 other boards and matches your sail types well too.
The smaller Jag 108 @68cm width is too close to your Blast 115. Falcons are for sure also a good choice, but then the 121 would make more sense width wise (75cm) - but not sure if you want to have a 100% slalom board compared to the other 2 in your quiver. Personally I think the Jag would suit the style of sails and other 2 boards better, no need to buy fins (Falcon supplied without), gybing and planing are easier, as is control on the Jag? hope that helps!