While it shouldn't be the tagamet, I have learned to rule out nothing and often suspect the unsuspected
The fact that it grew before quickly, maybe it's jut at that stage and the immune system needs more help. I had a girl who could not tolerate tagamet because it made her fllank swell where she had a chronic vaccine reaction.
I hate pred, but it has it's role. If I thought I was losing ground, I would try pred. And then I would try to get it to the lowest dose possible. It's tricky, since high doses might just stomp this thing down, but if you need it for maintenance, you want to stay low, so it will be determined by response of the lumps, how it's tolerated, etc.
I might stop the tagamet and make sure I'm using a large enough dose of the benadryl. If I'm still seeing growth, then consider strongly the pred.
In that thread, I a boxer, a breed where these tumors are common, multiple, and tend to be less aggressive than in other dogs. So would have to be careful to compare and think if all those things worked in that dog, why won't they work here? I also think they used pred at some point for a bit.