1. Unless you seldom watch big budget action/thriller movies, I'd put the sub in the corner next to the window to get maximum boundary reinforcement. The location where you have currently have it has zero of that, so unless it's some 2000 watt, 15-21" monster sub you are probably getting very weak bass which is especially unfortunate given that all your speakers are in ceiling which usually means they produce very little mid-bass already. What make/model sub do you have?I'm open to switching the system completely but priorities would be no painting/drywall damage, no random wires running all over the place(not ideal with the receiver all the way in the back of the room).
2. Wireless speakers are still prone to a lot of glitches. If you want to use in-room speakers I'd find ways to hide wires along baseboards, at least for the critical front 3 speakers where it will make the most difference.
3. If you have no interest in Atmos, I'd keep your surrounds in-ceiling. If you wanted to upgrade them, I'd look at the RSL C34e due to it having angled tweeters AND angled woofers.