The water pickup on the side should not be a problem, they are if in front of a transducer.
I had a B164 transducer on my RIB and he was reading up to 35 knots, those flush mount transducers in general read very well.
A fairing block has two purposes
1° to be able to cut it to the correct angle of the hull so that transducers surface is horizontal to horizontal sea bed
2° to divert bubbles to the side as bubbles usually glide attached to the hull to the side and put transducers surface above them
Was your transducer not reading above 6+ knots?
You have not many options,
a) another thru hull on a fairing block and cute better and let stick transducers surface more off the hull
b) a flush mount transducer as you think about and install it either in the existing hole or glass it close and drill another more distant from the water pickup
Keep in mind that the SS75 has a wider stem, so you will need to drill the original hole wider, verify that!
You can find the specifications and stem diameter here:
https://www.airmar-emea.com/Product/SS75C-0-M-MM
Your HDS Pro should be able to run a twin frequency band 1Kw transducer what would give you much better readings, even at 200ft, but the same kind B165/175 series is single frequency band too. If you are using the transducer only for knowing depth, but not for fishing then the SS75M is fine.
Chris