Nice cart. FYI a BOB stroller is perfect and has off road wheels. You can find them used cheap.
Love these for that very purpose:
Sport-Brella Versa-Brella SPF 50+ Adjustable Umbrella with Universal Clamp https://www.amazon.com/dp/B00HA2ZKFQ/ref=cm_sw_r_cp_api_glt_fabc_XRDTCRCN45E7RB5JXQS9?_encoding=UTF8&psc=1
Sport-Brella Versa-Brella SPF 50+ Adjustable Umbrella with Universal Clamp https://www.amazon.com/dp/B00HA2ZKFQ/ref=cm_sw_r_cp_api_i_ZxseFbGFJ0FB9
It has a clamp to attach- I put a pvc pipe in my fishing rod hole and clamp it to that.
Not an umbrella holder, but an attachable umbrella. Looks interesting...
https://www.amazon.com/dp/B00HA2ZKFQ/ref=cm_sw_r_cp_apa_glc_fabc_RSX3VBNZC3QH2155WGJG?psc=1
Yea! Welcome to the flotilla! Now, enjoy falling down the rabbit hole of the "Go Paddle" website and looking for kayak accessories. We just got some YakAttack kayak mounting rails for this season to mount cup holders and articulating arms for smartphones/cameras. For extra fun, when we're kayaking busy areas, we strap a bubble machine to the back. Don't forget the sportbrella to keep the sun off!
Anyway, ENJOY and sorry for the link abuse.