Thank you for the responses! You both offered good points to think about, appreciate.
Finances are always an important thing to think about and discuss. As for my situation, I am not worried about the financial part, it seems to be ok and steady enough. The travel expenses are something worth to be taken into consideration, though - especially if the traveling gets more frequent.
There are no children in the picture here and I do not want to have any, with neither of them. They both know this and accept it. Just as a comment on the kid schedules part: they tend to be challenging in all kinds of families, I have noticed.
What might become a challenge in my situation is the projects part. Mark has an old house, and it comes with an endless amount of projects. He is very used to completing them on his own, but I would love to be able to help him with them on a practical level as well. So far it has been more discussing them and giving him new points of view for making his best decisions. Of course that is an important part of partnership as well, but nothing compares to the physical presence and actually doing things together.
As for us, group living would not work. Both of my partners are quite introvert and need a lot of their own space. That is one reason why this works so well for us: all of us can get the space they need, the alone time. YouAreHere, I can understand your need for independence very well. Sometimes poly relationship can give the polyship members more independence and space than a monogamous relationship would.