You have had an interesting experience. I have an 8'8 Flash (custom), an 8'7 Placid, and an 8'5 Blurr V2.
I was surprised to read that you did not like the Blurr V2. I think it performs very well. However, I am 7-8 kg heavier (and 17 years older

), so it is my short performance board. That board is right at my limit - if it is too choppy/lumpy then I get frustrated. But on the wave, it is much more fun than the 8'7 Placid. In fact, due to some recent frustration, after 5 sessions on the V2 I took out the Placid and it felt like a boat. It was very stable, but I had to re-learn how much I needed to move my feet around to get it to turn. I fell off a few times just because the board did not go where I wanted it to go.
The 8'6 Flash is only 29.5 wide and 112L versus 30.5 and 121L so for you, the V2 should not perform as well. My Placid is 8'7, 31.5, and 125L, so you can see why the V2 is a performance board for me.
Anyway, I guess it depends on what you want in a second board. If you just want a little more stability, I would be tempted to go for a bigger Flash, just because you will likely be dialed in (I had a few Acid's for that reason). The Placid is good because it is still short but wider, so more stable, but go for the 8'3. It is a similar volume to your Flash but an inch wider. (Don't go for the 8'7" - it will be too big for you.) I haven't ridden an Allwave, but if it is also 8'3" I hope they aren't too similar.