hi

200 mg mag glycinate (or close to it) should be okay given the issues you're experiencing at the moment. caveat to follow.
recall, half of daily mag goes in *with* d3, ie morning in your case (still before food ideally), half the mag goes in on its own, eg as you suggest 15 mins before dinner.
when my mag status is good, and dietary intake high, 200 mg of mag glycinate 2x per day can be too much. i noticed that climbing the stairs was a bit more challenging. quads were a bit too relaxed.
normally i like solgar brand too, but magnesium BISglycinate has been trouble for me. too strong, and i was in the habit at the time of taking it before bed on an empty stomach.
glycine is recommended for pwms in other contexts, so i liked the idea of mag glycinate but i had a TERRIBLE year with mag BISglycinate. ended up in the hospital *again*. for whatever reason, it gave me terrible acid reflux, a really obnoxious cough, dry heaves, loss of sleep, weight loss. all because it sat in my stomach relaxing my LES. and it took me a year, towards the end spending the night sleeping upright in a chair, to finally figure it out. good times :S
it is that experience which informs the 15 mins before eating part of my suggestions. (the pharmacist was the one who said take half of daily mag *with* d3, half *on its own*).
i think others may be ok with mag BISglycinate, timed right, but i'm paranoid about it personally!
i am not sure anyone just coming off a long term high d3 bad mag regimem would have the same problem with mag BISglycinate in the short term, especially it it's not allowed to hang out in the stomach.
in my case, mag status had gotten so bad that it took months if not years before i wouldn't feel magnesium actually 'kick in' strongly after any pill, with something i ended up calling 'the magnesium knock'. a pronounced fluttering feeling in the muscles of my throat.
then it took more years before i could go a day without taking a mag supplement and not instantly feel spasticity return to my muscles. esp noticeable in forearms in my case.
i think worst case scenario with mag bisglycinate, would be as mentioned above that you can't manage two of them per day, and muscles get sluggish. safe enough to test out for a week. if taken before food sitting upright, GERD related issues should be preventable.
if 400 mag bisglycinate per day is indeed too much, you can switch half to a different form. in my case, after a decade of working on mag via diet and supplements, when i know i need to take two in a single day, i find using 200 mg per day of mag glycinate and 200 of mag citrate is a good combo.
if you find yourself exploring with a similar approach, you can mix things up a bit maybe. eg take your d3 with the mag glycinate or similar one day, and with the multi's mag combo the next.
i suspect 200 mg of mag BISglycinate plus 300 mg of mag oxide might be a bit much in a single day. possible laxative side effect issue? not sure. you could test it out for a week, see how you do. you could take the 300 mg one only on alternate days or something like that.
hope that helps!!