Baldur's Gate III's co-op is a great alternative to the singleplayer experience. But, if you can't find the time to play with your friends, and still want to roll on with the character you've made, you're at a bit of an impasse.
You can control your friends' party members if you go it alone, but you can't bring along the game's companions, characters like Astarion, Lae'zel, and Shadowheart. This is all about to change in the game's second patch, though, according to a post by Larian on the Steam community page.
Withers' Wardrobe of Wayward Friends will soon let you remove co-op party members in your campaign, meaning the transition between co-op and singleplayer should be much smoother soon. Many other changes will come with patch 2, so stay tuned for more updates.