IMO, longboard surfing is a different breed from shortboard surfing.
If you are after surfing more like the Kalama than your other SUPs, then you will have to stick with a longboard style board IMO.
The Speeed is not really a longboard style board, but it's a good board!
It is narrow, the rails are low and the deck is domed, so it is not a "stability" board when standing outside between sets.
It is a very stable board when paddling because the super kicked up, wide nose gives a ton of "lift" as you paddle and makes it rock solid.
If you are ok with a sort of "hybrid" ride, the 8'5 or 8'8 Speeed should be fine.
Speeed Guild Factors
Me at 100 kg (65 years old when I had the 8'10)
8'10 x 29 @130 L = 1.3 G/F
at 68 years old 107 kg
9'2 x 30.25 @ 148L = 1.28 G/F
Was my go-to NE winter board in full 4/3 mm wetsuit
You at 90 kg (age ?)
8'5 x 27.5 @ 113L = 1.25 G/F
8'8 x 28.5 @ 123L = 1.36 G/F
Hopefully with this info, you can gauge the size that will suit you best.
I just had my first surf on my Revolution 10' x 30 yesterday, but I didn't get much of a feel for it because it was basically all head high closeouts, where I'd get in one turn if I was lucky. The rails of the Revolution are super thin.... more like a prone longboard, so I found it fairly tippy for 30" wide, even with the wide nose. (75 yo 107 kg)
video of me on the 9'2 Speeed in full winter gear