Is there any pass to travel frequently between Brussels and Munich?

There are various sorts of season tickets, but those are for daily commuters.
I suggest you look at the "Bahn Card", website page here: https://www.bahn.com/en/view/offers/bahncard/bahncard.shtml
There are various versions, offering 25%, 50% or 100% reductions.
You would need to do the arithmetic to find out which option is best.
These cards only cover German trains, so you would (AFAIK - I haven't read the small print) still pay full price for the Belgian segment.

BTW, the company is DB = Deutsche Bahn = German Railways. I don't know why so many people call it "DBahn", it doesn't say that anywhere on their website.

The German Rail Pass is only valid for residents outside of Europe, and since you are legal resident in Europe, that would not work for you. The Interrail Global pass for European residents is 320 EUR for 7 trips in 1 month is the closest thing that would work.

The fastest trains would be 6 hours and 45 minutes with a connection in Frankfurt. I don't see anything cheaper than 90 EUR each way with Sparpreis tickets bought many months in advance.

I'd also check flight prices. If you can book well in advance, flights do not have to be expensive.

As Chris points out, you should check if a Bahn Card would save enough to pay for the card. Bahn Cards can give you a 25%, 50%, or 100% savings. The Bahn Card 100 is real expensive and only a benefit for someone, like a businessman, who does a lot of travel. Most Bahn Cards are an annual subscription, but I think there is still a Bahn Card "Probe", that allows you to try it for 3 months (?) at a greatly reduced price.

I just checked Google Flights for RT Munich-Brussels-Munich, nonstop leaving on a Friday in November and returning on Sunday and the RT price runs about $185. Fly back on Monday and it drops to $168. The flying time is about 1 hr, 20 min. Of course, that's not the total time of the flight, but it's less than the fast train and costs about the same.

Depending on your schedule, flying might be your best bet for saving some time and money. Lufthansa seemed to have the best options. Click here to go to Google Flights and check out the possibilities. Maybe your husband could sometimes fly to Munich?

Thank you everyone for the suggestions. Flight is definitely the best option. Plus I will also buy a Bahn50 card. The annual subscription is quite reasonable and benefits are good.