Probably much too late for your rat, but maybe it will be useful for someone else. I'm a rat owner for something like 8 years, I had a dozen of rats during this time, and my rats had quite a few operations.
Rats are small animals so you can't do as much healing and operating as on bigger animals like dogs and cats, but mammary tumor surgery is a quite popular procedure.
One thing worth noticing - the rat has to qualify for the operation (tumor location, tumor structure, age of the rat). Sometimes tumors are located in difficult places or have a difficult nature so sometimes the vet can decide not to do it, or recommend no to do it, because there's a danger to damage something on the way like for instance urinary tract or he will be unable to remove the whole tumor. We had two such cases, the vet warned us that there could be problems, but everything went well.
From my experience the operation itself isn't dangerous. My rats had dozen such operations and not a single problem. Some problems begin after the operation when rats try to work on the stitches with their teeth - the younger rat the healing process is faster and less time for them to open the wound again.
So doing this operation on younger rats like 1-1.5 years old is not problematic from my experience. 2 years is still OK, but the healing process is not that fast, so the whole thing is more problematic.
With older rats like 2,5 years old it becomes a hard decision, is it worth it? Should I do this to my rattie or just let him live as long as he can live comfortably with the tumor and then say goodbye and put him asleep.
Also, you have to know that the tumor can come back, and usually does, depending on a rat. We had a poor rat with new tumors appearing like every 4 months :(
From what we've learned through these years the best solution is neutering in young age (4-6 months) then tumors are much more rare.