I have sadly. My very first rescue cat that I adopted to my parents 15 years ago was diagnosed last November. It is a horrible tumour and they seem to appear and erupt over night. Sadly, at age 16 just 3 weeks ago my mom and I helped him over the bridge. (RIP little man).
It is a horrible tumour(s) however they are not painful but if they erupt there is dicomfort which is remedied with medication.
I wish you the very best. All I can recommend is to monitor the tumours, be aware of new growths and keep in touch with the vet on a regular basis which I am certain from what you write - you are doing just that.
My heart is so with you on this.