Hello,
Your S2S tunnels, are these with Check Point devices managed by the same SMS, or with 3th party devices ?
If it is a 3th party, this device will not know about the link selection for remote peers (Always use this address, statically NATed IP ) and can use your ISP2 IP address. You will have to put a host route to ISP2 for the public IP of the new S2S VPN.
For Check Point managed devices, ISP redundancy should be able to failover the VPN to ISP2 if there is a failure, but I don't think it is possible to point al new peers to ISP2 while all others have ISP1. ( Maybe changing the ISP redundancy settings and only pushing them to the new peers will work, but not optimal ).
What issues did you get with ISP redundancy ? ( we also got a lot of issues, but is is a bit stable now, only drops once a week )
Maybe Check Point SDWAN is a solution for you ?