you're right d3 is better than d2. so some cofactors are going in via the multivit. i did ask about the d3 not directly for energy per se but in case your d3 intake might be driving down tissue cofactor levels of nutrients required for energy.
i used to take 4000 IU d3 daily (so under 30K per week) with a little bit of daily mag, but the mag i did take was not enough. after a couple of years i personally ended up with breathing and swallowing difficulties. but mag has hundreds of things to do in the body so a deficit can manifest differently in different individuals. one of those hundreds of things is to cart ATP (aka Mg-ATP aka your 'energy currency') around your body.
when i was suffering the most, a pharmacist told me to take more mag and to time it better if i was going to stick with high dosing vit d. he said to ensure half the daily supplemental magnesium went in *with* d3, with the other half taken at a diff time of day, to allow for magnesium absorption for other purposes. it took me months before i didn't feel magnesium actually 'kick in' in my throat each time i took it, and years to restore depleted tissue mag to the point where i could go a day without a supplement, and not immediately feel increased muscle spasticity.
so that's my own scenario, but how each person's body uses mag depends on so many different interconnected factors that in your specific case, low mag could readily be affecting energy levels. not to mention that everyone is pretty much guaranteed to be low in mag without even adding d3 into the mix.
i'm surprised you need b12 injections every other week in addition to 3000 mcg sublingual b12 daily. what is the form of sublingual cobalamin? side question - any trouble sleeping at night with that intake? b12 and b6 work together on mood so it could be an idea to work in a b50 complex here and there, to keep natural ratios in line. i haven't seen any studies on whether loading b12 disturbs b vitamin ratios in the body and whether that can influence function, but i definitely *have* seen that when researchers are looking at the vitamin e complex. my fave ms diet/nutrient protocol (klenner - decades old and in need of an update) advocates for the entire b complex.
serum ferritin is an indicator of iron status, related to your blood's ability to deliver oxygen to cells. most common nutrition issue in general, even more so for premeno ladies whose daily iron requirements are more than double an adult man's. my daily high quality multi for some reason has no iron. last time (ie within last 6 months or so i would say) i went a bit overboard on vegetarian diet i started to feel pretty off and had to have someone bring me some floravit liquid iron supplement. i dislike it so much i just made sure to boost the iron density in the diet with more lentils, spinach and vit C, plus more servings here and there of things like clam chowder, ensure a serving of high quality red meat once a week (even if i sort of spread it out over more than one meal), etc. have been fine ever since on the iron/energy front.