How can you determine a conflict of interest if you don’t know what he’s been appointed to do?
If the role truly is advisory I wouldn't personally see that as a conflict of interest. Regulators are often asking for advise from those they are meant to regulate without it getting flagged as a conflict of interest (for better or worse).
Politicians and bureaucrats can and should consult with the people they govern of course. The "proper" way to do that would be via reasonably open and transparent process that is open to interested parties so competitors, customers, unions, scientists could have their say.
Again I'm fully aware this isn't how things actually work, so I'm not saying Musk is really doing anything outside the norm in American politics by buying a seat at the table. He's just being slightly more open about it than most of them.
From the FAA and Boeing to multiple health agencies and pharmaceutical companies, there are a ton of advisory type roles that involve industry leaders "recommending" policy. I'd be surprised if Musk ended up at the top of the list when sorted by impact, counted either by financial impact or number of rules and regulations impacted by industry.
Carter placed his peanut farm in a blind trust to avoid precisely that - sadly, we have seen a complete erosion of norms, standards, and morals in public life.