Side note on the exp share; earliest you can get one without trading is by getting one in Eterna City or thereabouts, if memory serves (or by being lucky with the lottery in Jubilife). I would also personally just go to the strongest route and switch Pokemon in and out if you don't have an EXP Share (ie weakest at the start of the party, switch when training), and if you meant that you are playing the same game file with the champion beaten and all, then the quickest way to train weaker Pokemon would really be just exp share plus taking on the E4 with Diamond, or rematching trainers on the Battle Island/near Sunnyshore.
Anyways, answered so going to close. If you have any other questions please use the Help thread sticky at the top of the section. Closed.