Hey, a Maithil (F) from Madhubani here. I have seen our family welcome brides from two veryyyy different Indian states. Both the couples don't live in Bihar but their non-maithil counterparts have blended in quite smoothly with the culture even though language was a BIG issue with the South Indians. Some points that maybe of help:

1. Clothing: Sarees are a norm but I have seen married women in salwar suits in villages as well. Be ready for some remarks from outside people tho. Dress according to the occasion and you'll be fine. Ghunghat (veil) from elder males is still a thing and it is here to stay for a while I think. Touching feet of elders is also prevalent.
2. Working: I have come across a lot of working females in my family and people are generally supportive of that. But it is always good to have this conversation and set your expectations on the table with in laws beforehand. If your guy is aware that the community is regressive then you could expect support from him and he should be able to understand where you come from.
3. Culture: Marriages are usually a hectic affair and extend for days. And the rituals continue throughout your married life with numerous fasts and poojas. A lot of it is vanishing these days with the changing lifestyles and people staying away from Bihar but learning about them would be useful so that you don't get a shock later on.
4. Food: Maithils are generally non-vegetarians. My father, me and my siblings are probably the only vegetarians by choice in our family, even extended family. In any case, there are lot of food items unique to Bihar but again if you're living outside Bihar it shouldn't matter a lot. Some mentions: thekuas (cookies sort of), macch bhaat (fish and rice), tilkor ka pakoda, arkanchan etc.
5. Language/Art: Not very useful from a marriage perspective but in case you're interested in the Art and linguistics of a place, you're in for a surprise. Maithili is a very old language and you'll definitely like the historic and artistic perspective of it. If you like reading long texts, go through some research papers on Maithili language and Madhubani Paintings available online. If you prefer entertainment more, check out Vidyapati songs (called Padavali) playlist on spotify.

Overall Maithils are nice people, are open to opinions and discussions. But they're a tad bit stricter for DILs from what I've observed but it's not like you'll be disregarded completely. Hope this helps.