migrating to new server, would like to keep sending email from existing server

Hello I'm in the process of moving my site to a new host (ie new IP). I run a social site that sends a large volume of (valid) email, and I have built up a good sending reputation with my current IP over the last 2 years.

My migration plan generally is :

1. Move site to new host/IP
2. continue sending site's mail from old IP
3. build up sending reputation of new IP : gradually increase amount of mail sent from new IP, while reducing mail sent from old IP
4. I would like my mail sent from both old and new IP to both be SPF and DKIM signed, and both appear as from [email protected] and return to [email protected]

Some info :

1. No, i cannot keep the old IP
2. Both servers are whm/cpanel based (which I am more comfortable with)
3. Site is php/mysql5.1

The solutions I looked at so far :

Solution (A) :

1. Set old IP as "permitted sender of" mysite.com and send emails from new IP to old IP via authenticated SMTP
2. The good : mail passes SPF and appears as being sent from old IP, nothing specific running on old IP except mail server.
3. the bad : mail does not pas DKIM, and sending from new IP -> old IP -> the world via SMTP is extremely extremely slow. I send 60k+ per day.

Solution (B) :

1. Set old IP as "permitted sender of" mysite.com and send emails from new IP -> old IP via remote SQL. Old IP then crons over any emails in the SQL and sends them out.
2. The good : mail passes SPF and appears as being sent from old IP. Faster than solution A.
3. the bad : mail does not pas DKIM, setup is a bit of a hack and not very maintainable.

Am I missing some very obvious and straightforward solution ?