Where should I start my career as a SWE/SDE?

Great question! Rahul and Alex already shared some of their thoughts on location here and here, and I mostly concur with them holistically that Bay Area in a vaccum is better.

However, I feel that you've provided some good context on what you are considering, and since I lived in Bay Area for about 2 years after graduation and moved up to Seattle (mainly because of my partner started her job there), I'll try to weigh in on this with a bit of my own personal perspective.

To learn the most, you need a good team.

From a professionally learning perspective, the most important thing is finding a good team for you to learn from and build your skillsets and perspectives.

With this in mind, the location doesn't matter as much as the composition of the team that's offering you a job. For example, in a situation where a team with most of its members in Bay Area and a minor hub in Denver, I would choose to be with the majority of the teammates in Bay Area, because learning in person is way more effective in person, early on in your carer.

Colorado is sufficiently big enough of a tech hub to have major companies like Google having dedicated engineering offices there. I think if you found a good team situation where most of the team is located there, you'll have a great opportunity to grow.

That being said, if you aren't yet at the stage where you are comparing offers, I would look at jobs at both location, if only to have potential offers from Bay Area as leverage.

Support systems are tremendously important.

Your career will have a lot of ups and downs. It's inevitable. I remember being part of the best team and under the best manager of my life for about a year; and one day, that was over thanks to an reorg. If you and your partner are serious, I would discuss whether Colorado is where you guys want to be long term, and make a decision together.

That being said, Taro isn't a ... life or relationship forum, so I'm not going recommend you to do it one way or the other. So I would say that if you do decide to go to Bay Area, make sure to build up your support network there (friends, etc) and keep the connection to your partner strong.

This decision is a 2-way door professionally

One more piece of perspective on this. Going to work at a particular location as a software engineer is a non-binding decision. In Amazon they call these "2-way doors". It means you can reverse the decision if you don't like the results, at least when it comes to your professional career. Going to bay area after 2-3 years in Colorado (or vice versa) isn't going to be very hard, and your software experience will translate no matter what location.

But some decisions are not. This includes decisions relating to your partner.

Personally, when I had to make the decision, I asked myself - if my partner and I were to break up because we didn't try to make it work being together in the same city, would I regret it? For me it was a resounding yes and that's why I moved.

Personal life comes first! If you don't have deep ties to the Bay Area but you have them elsewhere, I don't think it's worth it to move to the Bay.

Careers are long and flexible - Even if you don't find the success you want now, you can always grow into it later.

The same is not true for personal relationships. Trust is extremely hard to earn but super easy to lose. If a relationship falls apart, it's near impossible to bring back.

All in all, I'm more or less with Kuan on this one. Prioritize your partner. We work to live, not live to work.