This is longer than intended, but short version, if you are any deeper in crossovers than ROM v Transformers, Revolutionaries (while entirely optional with only small impact) should be your next step. Transformers bias, Transformers author John Barber, Kup is in every issue, some issues are heavier on Transformers than others.

The only thing I'll add, which is optional because the only thing it builds up to is First Strike which isn't Unicron related, is Revolutionaries (not Revolution, they are different books). Each issue has focus on a different franchise but with a core cast of

• Action Man (from Action Man)
• G.B Blackrock (from Transformers: Robots in Disguise)
• Ayana Jones (a member of GI Joe who only ever existed in Transformer books and was originally EDC)
• Kup (you know the one)
• Centurion (later in the series, new Transformer character)

If humans count, you might notice a bit of a franchise bias there. Plus it is written by Robots in Disguise author John Barber. I personally love the series, but if you already dislike Blackrock and Jones, you aren't going to lose much skipping it.
If you are buying single issues and just want to pick and choose, which at two paperbacks long I wouldn't recommend if you are reading it at all, but again isn't important for what you want, then

• Soundwave (#3) alongside Adventure Team
• Predacons (#4), yes they combine. Alongside GI Joe characters
• Transformers: Hearts of Steel characters, Shockwave for one page (#5), other characters spread across franchises
• Skywarp (#7) as a member of GI Joe. That is a thing.
• Optimus Prime and Metrotitan (#8) to a lesser extent Skywarp as part of GI Joe + cameo by Onyx Prime

 

I'm still debating whether or not to even read IDW's comics, as there is a new universe starting. The current universe seems to be a mess of crossovers, but even if I were to buy everything, I would go bankrupt due to how fucking expensive trade paperbacks are. And just to give you a clue to how behind I am, I so far have read Transformers: The IDW Collection volumes 1 and 2. I might need to take out a loan in order to get the rest of phase 1 and also 2.

 

There is a decade of stories before they ever did a crossover event. Of COURSE you should read IDW.

 

I'll probably read it all in the future, but I will probably focus on the upcoming reboot for the time being.

 

Completely agree. In fact, I’ve read quite a bit further than that but still feel like it would be incredibly expensive to get caught up. I thought about going the TPB route because they present everything in the correct order, but you’re absolutely right that that would cost a mint.