I’ve started a similar process to yours and am moving domains as they come up for renewal, with a slightly different technical approach:
I’m using AWS Route 53 as my registrar. They aren’t the cheapest, but still work out at about half the price of Gandi and one of my key requirements was to be able to use Terraform to configure DS records for DNSSEC and NS records in the parent zone
I run an authoritative nameserver on an OCI free tier VM using PowerDNS, and replicate the zones to ns-global.zone for redundancy. I’m investigating setting up another authoritative server on a different cloud provider in case OCI yank the free tier or something
I have one .nz domain which I’ll need to find a different registrar for, cos for some reason route53 doesn’t support .nz domains, but otherwise the move is going pretty smoothly. Kinda sad where Gandi has gone - I opened a support ticket to ask how they can justify being twice the price of their competitors and got a non-answer