Hi gazza,
You don't want flex in the trailer as this will transfer to the boat. With the design you have chosen you will certainly have some as you are going single axle with slipper springs I think. Tandem with rocker/rollers would certainly be beneficial but we won't go there.
The photos below shows the front draw bar tied into the trailer and you will also see that the keel roller frame is then also tied in so the trailer becomes one unit. You don't need what I have done due to your load being much lighter than what this was designed for but gives you a general idea. 90% of the weight on this trailer is taken on the keel rollers.
Just remember you will be strapping the boat down to the trailer and if the trailer is flexing you will be out fiberglassing again.
From memory I don't think you are towing the boat far and on good roads which should not stress the boat badly. If you on sell tho the new owner may have different needs and this will then become their problem if you don't get the trailer right. But that's not to bad as for many years I made money out of fixing boat trailers.
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