Well I can't say that I know every island yet but for me to grind exp easily in Akala Island will be at the Paniola Ranch where Miltank and Tauros is there, both of them grants a lot exp with the boost of Roto exp maybe you can max as 5xx I think? And for the second question, the decent Pokemon you can get over there will be two of my favourite, Araquanid on Brooklet Hill and Espeon on route 4 and 6 if you do the SOS grinding, anyway good luck and enjoy your adventure Trainer!