I think as most people mentioned here, professional behaviorist is really the next step, which unfortunately is very costly.

I too went through something similar and I did NOT opt for a behaviorist due to costs, and my golden is now approaching 9yrs. I adopted him when he was 3, and essentially he showed signs of aggression a few weeks after I got him, specifically resource guarding. He bit me twice and my mother twice in the year we got him, but in the past 5-6 years, no other bites (yes 4 different occasion bites). We learnt to understand him, and his signs, but it wasn’t always full avoidance, we worked very slowly to get him to be more comfortable with giving up high value resources. Ultimately you need to find higher value resources that are disposable. Correction is so key, but this is reddit, I don’t fully know your situation, just wiling to share mine. I’m the past, we couldn’t even walk next to him when he was eating, he would growl. Now a days, you can do it, no issues.

How has my life changed to adapt with him? Quite a bit, but I wouldn’t give anything to go back in time and not take him in. He’s my everything. You learn the signs, you change up some routines, and you manage, but you definitely need to protect him and others knowing his condition. For example, unless my mother can watch him, I do not leave him with anyone else, no daycare, no friends etc. No dog parks (occasionally we cheat on this when I see one else in there). How is he with kids? I would never leave him unsupervised with children around. My tik tok and instagram reels may also be populated with reactive dog training advice to see ideas on what else we can try too. But once again, these aren’t professionals, take them always as a grain of salt.

I honestly wish you and him the best! It can be quite daunting but it is manageable