In terms of pacing and stakes, it would have made much more sense for the PCs to have gone to Baldur's Gate earlier in the game to do all the "adventurers faffing around" stuff, then revisited the city during the endgame. Though it would have clashed with their "each act is one set of maps" setup.

Instead, in the last act we have Gortash, supposed 5D chess player, centering all his plans on the PCs flipping to his side. Then he sits back and lets them wander all over the city, undermining him. Ultimately, when they don't take up his offer, his backup plan is "whelp, guess I'll die".

Maybe the excuse is that the Elder Brain was making him stupid...