Thank you for the suggestions and continued kindness. It is very much appreciated, especially considering what we've been going through. While I do know that a grade 5 heart murmur is serious, does anyone have experience to share about their own dogs having such a serious heart murmur? Is there anything I can do to help keep him as comfortable as possible? I wish it were easier to take him for rides in the car. Nothing seems to help- whether his other parent (my partner) is holding him, he's in his crate, or laying in his bed, he gets himself very worked up.
I also feel like this is my fault, because I didn't realize how serious a heart murmur could be

I took him to the vet in September of 2019 to get a check-up and senior bloodwork done. They told me that they might have heard a faint heart murmur, but when they called in another vet to verify, she couldn't hear anything. They suggested I follow up with them, but had also told me a possible mild murmur wasn't something to be alarmed by, at least not anytime in the near future. His bloodwork results were excellent at the time, and he had no other signs of illness. Fast forward a few months, and we moved out of state during the beginning of Covid. I couldn't find a vet that was accepting new patients, and I felt bad that we couldn't be in the office with him due to safety protocols. It only took a year and a few months for his possible heart murmur to become a grade 2 or 3

I definitely know better now, but I'm going to regret this for the rest of my life.
That vet that I last spoke to made it sound like it was probably "pointless" in her eyes for me to keep trying with him. However, the vet who actually saw him when we were in the office made it sound like he could stick around with us a little while longer as long as the meds are helping him. I'm trying my best to consider his quality of life, and find the lap of love quality of life scale to be helpful for doing so. My partner & I want to give him more time, and try adjusting his meds when necessary before letting him go. He does still have good days, and good moments. He is still eating and drinking. He still wags his tail at us, and still wants to snuggle and give kisses sometimes (he has always been a little moody from time to time, even when he was a puppy!). Donovan is a fighter, and as long as he's still fighting, we're going to do whatever we can to fight for him.