Hey! I own MilkBox. I think here is not a good place to drop a pitch, so I'll keep it holistic/non-salesy.
It's a pain in the ass to set it up yourself. High cost in terms of time/effort (even if you hire a VA, you still gotta check everything). You are the one who needs to fix every issue.
I haven't set up mailboxes directly with Google/Microsoft since 2022/2023, so IDK if there's a difference in performance now. Back then, I AB tested it and saw no difference.
Across providers, there's definitely a difference in performance (reply rates) and management quality. Google is usually good for lower volume & Microsoft setups are usually better for high volume, as they are cheaper per inbox. I'd avoid custom SMTPs since you're a beginner - but if you do use it, use a mix of multiple providers to diversify the risk.