How to set up multiple DKIMs on different smtp servers?

Question

SPF

There is mail on Yandex, spf record and digital dkim are implemented. Yandex has a limit on sending letters per day, which is not entirely satisfactory. There is a server on it to spin the configured e-mail server, sending from this server is resolved by spf record and passes check, but unfortunately dkim=fail. On the dkim mail server, the signature is also implemented and works for domains that are initially associated with the mail service.
Yandex spf record defines as softfail supposedly ip v6 address is not a valid sender, although Google sees the same host as ip v4 and spf=pass for example. Naturally both addresses are present in spf records.
I've been scratching my head for a couple of days now, I need your help.

Answer 1

SPF softfail - Add an ipv6 address to the SPF record.
DKIM - take another selector, generate your own keys for it and place the corresponding entry in DNS. Then letters from Yandex will have their own signature, and letters from your server will have their own.

Answer 2

Did you solve this problem?