Employees, Partners, Investors, Execs, etc. They are all humans, with a perspective, purpose, and preferences. How do you make sure people remain driven towards the startup's vision and constantly rooting/supporting each other?
The first thing you need to do is make sure you are aligned with your partners and investors before you agree to anything (or to move forward). That doesn't mean 100% alignment, but you should all be heading in the same direction. You hire employees, so you have a lot of control over making sure they align with the mission and vision of the company before hiring them.
Support can come through encouragement, peer pressure, or incentive. Group incentives will use peer pressure to keep everyone in alignment.
If you are having conflict or misalignment between constituency groups, consider hiring a mediator or facilitator.
It depends on what you mean with "align". If you mean all agree, then you will not get there. If you mean not raise issues you have missed that impact the vision, mission, and ambition, you will never get there. If you mean 100% support what ever you believe is the company direction, you will never get there.
And neither should you seek such. None of us is as smart as all of us. Having been in every role you describe, radical candor and perspective have been critical for success. The difference is healthy and productive disagreement not toxic and divisive disagreement.
I recommend every startup read everything they can about Amazon, most critically, the Leadership Principles. There is likely no modern company that executes as strongly as they have while retaining, at their size, an environment of innovation and discovery. Every person that hopes to work in a professional capacity is required to apply these in their interviews, and will certainly be judged by the conversation because they believe that cannot afford to have misalignment. [Full disclosure: I live in Seattle and have many friends at the company, everyone impressive and focused.]
You solve people problems by understanding what's at the root of the problems, typically a lack of leadership, no skill in hiring, and poor communication of values, goals, standards, expectations, and vision for the company.
There are six fundamental business skills that every company will need. They're not the only ones, but they are the foundation. Marketing, Sales, Organization, Efficiency, People, and Leadership. No one person will ever master more than two. A business owner must work to become aware of what goes into those skills they lack in order to know if those supporting them are doing what they need to do.
Work to understand which skills you have mastery of, and look for advisors, employees, coaches, partners, service providers, etc. who can fill the skill gaps masterfully. No one expects you to be talented at everything, but they do expect you to know when to bring in what kind of help and support.
So no, PEOPLE is not the biggest problem, but it might be one of your very weak areas. There are things you can do to improve, but remember you'll only master 2 of the 6. Plan for and ask for professional help where you need will it.