TBH I do not know how much magnesium is too much if there are renal issues because my interest is not in that field. I suggest that you research it because you are likely to find recommendations. It seems like you are only guessing about renal issues which does not really help.
The only thing we really know from what you have said is that you are constipated. I would say that you might very well have leaky gut which does tend to cause constipation because the liver uses up all available choline which leaves a deficit when it comes to acetylcholine production to stimulate the bowel. Is there anything which might indicate that, such as anything even remotely autoimmune related?
There can be other contributory factors in constipation which leaky gut can make even worse. I should know as this affects me too. Some people, strangely water signs almost exclusively, can tend towards constipation when they suppress sad emotions. This is so common among many people but in water signs ie cancerians, pisces etc when they suppress the expression of sadness it affects the nerve stimulation to the ascending colon (right side). If a person also suppresses the inner sad feeling then it also affects the nerve impulses to the descending colon (left side). Both combined will cause a very slow bowel. If leaky gut combines with those then severe constipation results.
To see if the emotions affect you, you could look into your irises and check inside the colon for a faint shadowy ring. There are always shadowy rings that run around the iris which are produced by blocks to drives and emotions. If one exist from the 10 o clock to about 6 o clock in the left iris and 2 o clock to 6 o clock in the right iris then these will cause a slow bowel.
BTW the stomach and colon are inside a ring which usually occurs at about 1/3 the way from the pupil. Use a bright light when looking to contract the pupil. A low power magnifying glass of about x3 placed against a mirror can be helpful.
Challenging the beliefs picked up during early childhood can eventually eliminate these causes and allow the rings to dissipate and nerve impulses to the bowel to work fully. Magnesium can help that.
I do not believe in astrology, it's just something that I noticed with regard to water signs. The emotional repression is a key element that makes such individuals affected in this way.
I have such rings inside my ascending and descending colons. I have released them quite a bit but ATM they are still active ie I have not yet completed the elimination of the beliefs picked up from my parents that create them. It's an ongoing process to eliminate them.
I use quite a lot of lecithin along with small amounts of magnesium citrate. I get plenty of magnesium from other sources so magnesium citrate is only used to help my bowel a bit. The lecithin has many benefits to the body besides helping bowel movement as it is used for cell membranes and helps the liver.