Theres so much to talk about on this subject ie longevity of life it can come across as incoherent, its very much a case of where do you begin, because the human body is very complicated. The interactions between medicines you may be prescribed by your Dr is not even complete.

I did an experiment a few years back working on laptop outside in just shorts during UK summer. I got my levels up to just under 500nmol/L (478nmol/L iirc) verified by the Vit D centre at the NHS Birmingham and Sandwell using their blood spot tests, sending one of their bloodspot tests to them a month. http://www.vitamindtest.org.uk/vitamindbackground.html

200-220nmol/L is considered the safe upper limit, 500nmol/L is considered toxic, I was warned if I went higher I could get sarcoidosis which is granulation of the lungs amongst other things. However the NHS medical person who has a PhD in Vit D that worked there, who I had a lengthy conversation with on this topic didnt know the role iodine plays at killing cells in sarcodosis, only hypothosised in Greece in 2013 and didnt know about the 1937 study of how water washed off pre-vit D from the skin.

Likewise if you get too much 25(O)HD circulating other risks to health include hypercalcemia, ie too much calcium circulating in the blood which can lead to things like stones and other problems.

As Vit D increases the absorption of calcium from the diet/gut, you then need the fat soluble vit K to move the calcium from the blood to the bones as Vit K increases oesteoblasts (bone building), or increase iodine so the thyroid can release more calcitonin (amongst other hormones) which gives you a similar effect to Vit K. Your diet also affects what you produce as well, my diet is generally ketogenic. Your liver produces around 80% of the cholesterol which is needed to become Vit D. In fact a study in the UK done sometime in the 50's showed British gardeners had lower cholesterol levels, so you can lower your cholesterol by getting some sun. Of course if you have damaged your liver through excess alcohol, smoking or drugs then your liver is not working at optimum and thus you need to increase your dietary cholesterol intake whilst also increasing your consumption of B vits to help the liver regenerate as its the only organ that regenerates naturally and this is how they used to treat people with liver cirrhosis in the early 1900's.

As mentioned above with Vit D, theres a few different forms of Vit D and Vit K to name a few, in fact one Japanese study showed Vit K2 MK4 is used to treat post menopausal women for osteoporosis but then reducing vit k is used to thin the blood to prevent heart attacks. Fat soluble vits have a have life in the liver, Vit A has the longest half life, followed by vit D, then E then K with some forms of Vit K only lasting for a few minutes to hours to a few days depending on which form of vit K it is.

Anyway whilst I have seen studies to suggest the Vit D is rate limited by your body, I proved its still possible to get above what medical experts consider the natural limits in the body. You see melanin the skin pigment/tan converts 99.9% of UV radiation into heat, the skin cancer risk comes from burning too much, you want to build the tan little and often to maintain the tan all year round, but melatonin whitens the skin. UVA reactivates melanin, UVB creates new melanin. Copper stored in your liver is oxidised to become melanin. Likewise if you dont have enough Vit A your skin wont produce enough oil which becomes UV radiated and is where you find the pre-vitamin D, likewise showering even in just water every day washes off the pre-vitamin D, so I dont agree with some of things Hollick (arguably the worlds leading expert on Vit D) says, plus there is the cholesterol angle to also help increase Vit D. Your brain is made up of a large amount of cholesterol as well, which is why some studies have shown some statins make people forgetful when on them. If their liver is not producing enough cholesterol, how can your brain keep working properly?

So, yes I can agree with you that there is going to be a rate limit to the amount of Vit D that shows up in blood tests, but depending on the type of Vit D test you do, depends on the levels it shows. You see different manufacturers who make the machines which calculate the levels of Vit D in the blood use different protein markers, so theres now a group underway to standardise the levels across all the different machines and methods used. More info here. https://ods.od.nih.gov/Research/vdsp.aspx

> Additional sunlight would not materially increase circulating D level.

Due to pollution, we get less sunshine today which has also been spotted with things like some frogs not reproducing as much today compared to just a few decades ago such is the rise of atmospheric pollution, global dimming et al, but you need the sunshine to start the process or get yourself some of these to lie under like a lizard. ;-)
http://www.uvguide.co.uk/lightingsurveyintro.htm
Its also interesting to note that UV levels do vary around the world as noted here. http://www.uvguide.co.uk/uvinnature.htm

If you live in a town or city, vehicle pollution reacts in the sun to create ground level o-zone which blocks uv-b, yet 100% of uv-b is reflected from man made surfaces like concrete so whilst you can get the best exposure to uvb in a town or city due to the manmade materiasl reflecting it so well when compared to plant matter, its restricted by the ozone pollution. In the countryside around 30% of uvb is absorbed by the plants, yet due to little or no pollution, realistically you get more uv-b exposure being in the countryside even though plants take around 30%.

Like I said, I have read alot of studies and having a high signal lesion in the genu of the corpus callosum which I found out when I had a MRI on my brain also helps retain this data.

Studies have also shown high levels of Vit A and D enough the pancreas to start producing islet cells so you can have expensive stem cell therapy if you are diabetic or if you cant afford that get you A and D levels up. All of this info and more can had for free by reading the studies you find when using google scholar.

To quote Thomas Pynchon, if you dont ask the right questions, who needs to worry about the answers? Same applies with google and other search engines, if people have an inquisitive mind.