I don't have any experience with the Buspirone, but I am on Mirt and Effexor. I was in protracted withdrawal from Effexor when I was put on mirt. It helped me sleep, but very quickly it wasn't helping otherwise and so I kept upping the dose until I was at 37.5 mg and feeling worse than ever. I knew giong higher wasn't going to work, so went back to my doc in desperation. He thought since I had tolerated Effexor well in the past that maybe we ought to try it again. Well, that worked, but because I was getting the fix I needed from withdrawal!
So now I am on both, but tapering. I have learned tons about withdrawal and these drugs in general.
Mirt goes well with SSRIs and SNRIs because they work in complementary ways. Not sure about Buspar, though, as it is not an SSRI. I would be a little concerned about this combo because you are on a high dose of mirt already. I did a drug interaction check and found the following:
Interactions between your selected drugs
Major buspirone mirtazapine
Applies to: BuSpar (buspirone), mirtazapine
Using busPIRone together with mirtazapine can increase the risk of a rare but serious condition called the serotonin syndrome, which may include symptoms such as confusion, hallucination, seizure, extreme changes in blood pressure, increased heart rate, fever, excessive sweating, shivering or shaking, blurred vision, muscle spasm or stiffness, tremor, incoordination, stomach cramp, nausea, vomiting, and diarrhea. Severe cases may result in coma and even death. You should seek immediate medical attention if you experience these symptoms while taking the medications. Talk to your doctor if you have any questions or concerns. Your doctor may already be aware of the risks, but has determined that this is the best course of treatment for you and has taken appropriate precautions and is monitoring you closely for any potential complications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.
Hopefully, your doctor knows what he is doing in this regard, but I would recommend doing a slow taper off of mirt since it wasn't working for you and best to not be on more than you need.
However, mirt is very tricky to get off of, and the taper schedule that most doctors advise is way too fast and will cause withdrawal, even though you'd be on another med that might temper that a little.
If you would like information on how to taper safely, visit the topic "Reducing ADs using a 10% withdrawal method" in this thread:
https://patient.info/forums/discuss/depression-resources-298570