Reagan's coin was already authorized under the original legislation for the presidential dollar series (nothing in the legislation said they can't skip a living president if the president after him was deceased. I remember arguing this with people until I was blue in the face with people who insisted otherwise, but well I was proven right eventually lol, so there). Officially the series ended in 2016 with the issuance of the Reagan dollar. A separate piece of legislation authorized an additional one for George H. W. Bush to be minted in 2020; technically the language of the legislation made this a separate coin issue, but it was chosen to keep the design elements so it fit with the rest of the 2007-2016 presidential dollar series. It would take a separate piece of legislation to authorize one for Carter, but the precedent has been set with Bush so I imagine it will likely happen 2026/2027 or so. I don't think there would be much pushback in just eventually authorizing every current and future president 2 years after they die, so this series may effectively go on indefinitely. The Bush dollar already set the precedent. I can't imagine what arbitrary reason anyone would come up with that it should stop for any president that served after George H. W. Bush. If they're going to honor some, may as well do all.
Well I was only 5 in 1981 when they were discontinued (not counting the 1999 issue) so I only have other people's word for it. It effectively was discontinued (not counting 1999) before I even knew it existed. I got into coins when I think I was about 8 in 1984. (People can do math to figure out how old that makes me lol, I'm fine with that.) They only did one for FDR. Grover Cleveland got 2 because of his 2 non-consecutive terms; it's a traditional convention to call him the 22nd and 24th president, just like Trump will be called the 45th and 47th. Other presidents that served 2 consecutive terms did not get 2 each. No comment on Elon Musk except to say he's a naturalized US citizen and thus not qualified to run for US president unless the Constitution is amended.