My condolences. Losing a pet is so hard :-(
You didn't just do the right thing, you did the only fair thing.
My child was diagnosed with a BT as a toddler. She's mostly ok now, but it's taken years of recovery, and she was very fortunate in that it was a type that wasn't aggressive. But HUGE surgery, complications, years of physio, prolonged rehab.
And in the weeks before she was diagnosed she was sad, hurting, clingy, emotions all over the place.
We don't know what dogs feel, but it's not a leap to suggest doggy was feeling similar.
Your options really were potentially huge neurosurgery, from which doggy might never fully recover, and that doggy wouldn't understand, and depending on the type of tumour, possibly died anyway. Or do nothing, and it would have gone past how my little one was into genuine suffering until either a long death or a stroke. It would have been horrific for you and your dog.
Even if curable with surgery, I wouldn't put a beloved pet through what my child had. They don't understand the future like we do, they wouldn't understand why their brain didn't work properly, or why they were unbalanced, couldn't run etc.
Giving doggy the chance to die without too much pain, without the pain getting really bad, is the kindest thing you could ever do for your dog.