Leave theRhumney River as early as you can and sail straight across,You can get onto the club pontoon on most tides an hour or two before/after highwater on Neap tides you need to be there at HW. On big spring tides the walkway covers leaving you cut off from the shore unless you take a dinghy.

The other option is to leave RRC after HW wait off Penarth or Flatholm as Snoopy suggested then go in on the next tide.No point entering the river earlier than two hours before HW.

At the other end of the Bay you can lie alongside the "dolphins" in Knightstone harbour easy with a bilge keeler.
Knightstone Dolphins

WBYC pontoon

river Axe

You need about 11 meters of tide to get into Knightstone.