Toxicology and testing methodology is above my pay grade, but if I'm still carrying any level of barium after 45 years then I worry about the cumulative burden of it
Maybe it's not the same barium atoms. You may have lost effectively all the barium from the x-ray, but eaten some Ba-rich food recently. The problem with ultra-sensitive tests is that the interpretation of results may lag far behind the sensitivity. If fairly insensitive tests 50 years ago came with a 'safe level' of 1 mg/kg of body mass, what does a more recent test showing .1 mcg/kg mean? By old standards, you're practically barium-free. If the (paid for private) tests results is displayed with bright red bar graphs, they might convince people to buy their chelation services.
'Barium that enters your body by breathing, eating, or drinking is removed mainly in feces and urine. Most of the barium that enters your body is removed within 1–2 weeks. Most of the small amount of barium that stays in your body goes into the bones and teeth.'
Also: 'However, information is still being collected to determine if long-term exposure to low levels of barium causes any health problems.'
For methymercury: 'The half-life period of methylmercury, that is, the time in which the content of methylmercury in the body is reduced to half through excretion, is 70 days on average.' so, if you do ingest a significant amount (typically from fish), you can work out how much will be left x days/months/years later. Also, you have to clearly define 'safe'. Yes, a single molecule of methylmercury can damage a tiny part of the body, but that doesn't mean that a single molecule will noticeably harm your health. Saying that 'there is no safe level' is true, but rather meaningless. There will be a level that is 'damaging a few cells, but not having a noticeable effect on your life'.
There are a lot of health scares based on misapplication of perspectives.