Hi Eoin,
I totally understand the concerns when migrating from Lotus Notes to Office 365.
Currently Office 365 doesn’t support the migration type officially. However we have some discussions in our forum regarding the issue. For details, please refer to the links below:
Third-party mail migration to Office 365 -- fixes and tips
Move Lotus Note to Office 365 recommendation
Migration from Lotus Notes to Office 365
Lotus Notes migration tools for Office 365
Also we welcome our community members who have done this before to share their experiences in the migration. I’ll monitor this thread for a few days in case if you need any help.