Ok, it will be very simple.
But first let me address one possible misconception:
This is not true, The internet is designed so that each part may run independent of each other. Even one.com respects this though they do not so state in their support articles, but a chat with online support confirms everything.
So you do not NEED to transfer the domain to gandi, but it will be beneficial, and it is a slight bit less expensive for renewals. One.com costs $20.50 per year for renewals and Gandi is $18.96, but Gandi offers free Secondary DNS service, which is very important.
The first problem is an egg before the chicken scenario. Do we transfer the domain first, or last? I am going to suggest that the domain be transferred last.
So the process will be as follows:
Set up Glue records with one.com
Set up MiaB
Create the required mailboxes on MiaB.
Transfer all emails using IMAPSYNC.
Update the NS records at one.com to point to MiaB. (You’ll have just one name server temporarily.)
Wait x time (the TTL of the MX record at one.com) and redo the imapsync to pick up any potentially missed emails that arrived during the transition. (As you mentioned having 12 mailboxes, you may prefer to completely stop all incoming emails before you begin the migration. This becomes tricky though due to the timing of the TTL, so I recommend doing a second imapsync.)
Initiate the domain transfer from one.com to Gandi.
If offered by one.com complete the expedited transfer process, If not, wait the 5-7 days.
When the domain is transferred, add ns6.gandi.net to the name servers at Gandi and as a Secondary name server on the Custom DNS page within the admin area of MiaB.
Enjoy your new MiaB.
Now it is possible, if you prefer, to transfer the domain from one.com to Gandi first. There may be some issues with downtime doing it this way though unless you set up 3rd party DNS first. So neither option is ideal but the one I laid out is more ideal.
ETA: I just had a very disturbing chat with one.com support and I hope that the agent is just absolutely mis-informed. They are stating that when you initiate a transfer ALL glue records, and name servers are deleted from the registry at the beginning of the transfer. If this is true, you will have a down time of 5 days. Absolutely NO other registrar does this as it is very malicious. You may want to consider registering another domain to run your MiaB instance on due to this malicious policy. Again, I just hope that this rep is absolutely incorrect.
So if a domain has glue records and name servers set, you remove that information before the transfer and do not leave it intact at the registry???
Those records will remain with us that’s why you need to recreate those records again since those records is not part of the transfer
Please feel free to reach out to me on Slack if you want to cover anything in more detail. Also please PM me your domain name so that I can check the TTL and give you a proper time that you must wait.