You are in a difficult position and may want to check in with your vet again. I had a cat, Thisbe, who went into heat every two weeks or less. We had to delay her surgery for a few years due to other serious health problems, and she did develop mammary and then lung cancer in her early teens. If your vet only wants to wait a few more months, that should have little impact, but you may want to tell her about the frequency. Snoopy is such a love and with so many issues, I'm glad she is in your good care.