General note: If unsure, start slow.
General note #2: plan your sendout increases so they suit the slowest of your domains and adjust the others to the slow pace, not the opposite. Spoiler: in your case, Microsoft will dictate your plan, not gmail.
+Rushing through this won't help you in case you need time to allow for remediation strategies to happen without impact on major sendouts.
With microsoft, a need for remediation has been a common pattern for us. While your sendout volume is low, you can monitor, potentially wait out a support ticket to resolve an issue, and increase later. If you rush to pump out large numbers through an instable system and it fails, first you might burn your IP, but also a large sendout needlessly increases the opportunity cost / business impact of emails not sent. (Why bother sending a lot if say, 95% goes to spam anyway) >> It is in everyone's interest to work slowly.
While all of the above is true, gmail in particular has been a no-brainer, typically other domains dictate the plan, Gmail just rolls along.
We have regularly kicked off an IP Warming with 20k on gmail. We have then doubled weekly and never experienced problems. Sorry for anecdotal evidence, but we do IP warmings every couple months, so I feel fairly confident that this is currently correct. With my general note #2 in mind, it doesn't really matter how high you can go. No need to trust my anecdotes, feel free to start gmail at 5000, you will probably anyway devise your plan based on your slower domain:
register your domain at SNDS https://sendersupport.olc.protection.outlook.com/snds/
to get a green / yellow / red kind of status update on what microsoft thinks of you as a sender on a daily basis.
We typically start off with 1500 addresses on microsoft, with plans to double weekly, or, if we can go for 4-7 mails per week, we double twice per week. This number is rather arbitrary, but again - it doesn't really matter "how high we can go" -
We typically see "red status" on SNDS right off the bat, which won't auto-resolve. It corresponds with suspiciously low opening & low click rates & mails that get delivered to the spam folder on some of our own microsoft inboxes we include. The solution is to enter a support ticket w. SF deliverability who resolve this with Microsoft. The support ticket typically takes a few days, during which we keep to our slow plan (as rates are anyway not great), and within <10 business days or so, we are "green". In this timeframe, it is pointless to cram as much volume onto your new IP as possible.
There also is a thing called Preemptive Accomodation where you can apparently alleviate this in advance though microsoft support.
As our approach above has predictably worked for us this far, I have not tried that. But returnpath should know what they are talking about here and it might open the door for speeding up microsoft, and gmail can start reasonably fast anyway.