I am currently in the process of upgrading the audio in my 2019-non ML.

Crutchfield is recommending that I mismatch my speakers based on my budget and prefered specs

• Center Dash: RetroSound R-452N - $99
• Corner Dash : Rockford Fosgate R14X2 - $60
• Rear Door : Infinity Reference REF607FSL - $99
• Front Door: Morel IP-UNI82 - $430 - (the only 8 inch speaker shallow enough to be an exact fit without a bracket?)

I am willing to break the budget if I can get a uniform set up. I am eyeing a set of Infinity Reference speakers as follows:

• Center Dash: Infinity Reference REF407F - $110 (REF307F as a back up if the 4 inch dont fit)
• Corner Dash : Infinity Reference REF407F - $110 (REF307F as a back up if the 4 inch dont fit)
• Rear Door : Infinity Reference REF607FSL - $99
• Front Door: Infinity Reference REF607FSL - $99 (I found an 8 inch to 6 1/2 inch bracket, but still let me know if there are other options)

I wanted to know what issues I might run into if I attempted to go with the 4inch model for the entire dash as I know that the 3 1/2inch speaker will just be slightly short of the mounting holes but the clearance will be just right enough that I could make a small extension bracket. The goal was a direct 1 to 1 upgrade with low mods required.

If no answers I will attempt the mod and report back what I find on here.

 

I got the 4" Infinity ones for the dash and I had to do a lot of cutting to get the speakers set deep enough to not touch the speaker grill/cover. There's plenty of depth in the dashboard to fit whatever, it's just the hole isn't wide enough. If I had to redo it I'd buy a snap-off utility knife to make cutting easier and the lowest-profile bit holder to hold a Phillips head bit to screw the speakers into their clip-on mounts. And I would've soldered the new speaker wires together instead of just crimps, but I wasn't as good at soldering at the time.
I didn't replace the center because the speakers are sold in pairs and I figured I'd replace it later once I thought of what to do with the spare speaker left over, but I've read it makes a huge difference. The infinity speakers are much much much brighter than the stock speakers.
I actually replaced the Rockfords with the infinity because someone on youtube said they were better and they're just a touch better to me.
I threw Infinitys in the rear door and they're pretty good. I notice them if I have my seat too far back.

 

I just today put in JBL Club 3.5 inch speakers in the dash. They fit very well, though they only one side will bolt in. I solved this by getting a washer to sandwich the other bracket between the stock washer plate and the bolt.