The proposed changes sound good so far. Personally, I watch the Transformers movies for the robot action, so if you can keep as much as that while cutting down on the fluff, it would be a huge improvement. Granted, these films are sort of guilty pleasures for me, but dangit they’re enjoyable!
There are also a whole bunch of continuity errors among other things in the films. Here’s a list of them for T4: https://tfwiki.net/wiki/Transformers:Age_of_Extinction(film)#Continuity_errors. Not sure if it’s worth attempting to address anything in here, but it may provide some useful information.
My suggestion is simply to cut down on the humans goofing around and wasting time. I personally think 2 and 3 are decent enough on there own, but I’d like to see how you’d combine them. Films 4 and 5… yeah, there’s a lot of fluff that can be cut out. Transformium? Really? All we need is a simple plot to keep things moving forward.
Also, about Optimus leaving Earth. In film 4, Quintessa (his creator) sent Lockdown after Optimus to bring him to Quintessa. Optimus wanted to find out why Quintessa wanted his presence, so he left Earth at the end of film 4. In the beginning of film 5, he just gets blamed for the loss of Cybertron and then subsequently brainwashed by Quintessa. Not very compelling, but it serves the plot.
If I make it so Optimus get kidnapped, and does not escape I can get 4 down to 54mins to have it be, Cade meets Transformers, show why Autobots are hiding, show Lockdown being a treat, show Megatron being reborn, no transformeum, and Optimus telling Cade to warn his Autobots. Because I can have it be like when you meet up with Prime he’s already with Quntessa. Like Lockdown has already handed him over to her. That way I have 2 hours, or less, to do LK’s story line, If I wanted to make this the same length as 4 or 5 originally was.