If I were to take this position, how can I successfully keep a good work-life balance while traveling so much? Is it even possible?
You can't, unless you can find things you want to do in the evenings (more SE?) which are not sacrifices from your current lifestyle.
By definition you will be spending more of your life energy/time dedicated towards work. I'd suggest Your Money or Your Life as an interesting book as you consider this decision. Work has a lot of additional costs, both monetary and time. Traveling like this adds a significant time cost.
However, "good" is an entirely subjective term. For some people good work life balance would be working less than 50 hours a week (doctors/senior management). For some in Europe it'd be working less than 40 hours a week and getting 6+ weeks time off.
That being said, you mention two things that make this incredibly hard are:
- Long term relationship heading towards marriage
- Your dog
Neither of those can be maintained without in-person presence. Only you can know how important that in-person element is (maybe your partner works evenings and is already unavailable during the week). And only you know what your out of work interests are.
For example, let's consider that you spend 15 hours a week reading books, playing video games, and consuming on the Internet. If you are disciplined enough to move that activity to your travel days and reallocate other things to your actual available time, this will be less of a worklife imbalance than if you cannot do so (or have a woodworking hobby that takes the time instead).
You might also consider any social circle you have. How important are activities you do outside of work? If you are gone every Wednesday and have a regularly scheduled activity you will miss, you are going to be giving this up for work. How often do you hang out with friends? Etc.
Additionally, for some people travel is exhausting. Some people are energized by it. This is a factor you should consider. Will flying 2x a week make you exhausted and have to recover a full day on the weekend? Or will you come home SO EXCITED to do everything? For me personally, this is a huge reason such an arrangement could never allow acceptable work-life balance.
Most of the people I have worked with who did such arrangements spent more time at work, too, because they didn't have anything really to do in the evenings.
Basically, make sure you consider the full implications that traveling this much has. Whether you can keep work/life balance depends on how important the things you have locally are and how difficult/exhausting travel is. Everyone will have different perspectives on this.
For me personally, I would need a huge amount of money to make it worthwhile to be gone from my family and friends that often (given alternative options).
Also, you might consider suggesting relocating instead if this is a big factor. Hard to say how feasible this is, but it might be an option to consider.