Should I abandon any further higher education knowing that I was dishonest as an undergraduate? This feels uncomfortable to ask, because I know my actions have undermined what I've worked toward.

A little background: I graduated with a bachelors degree, and then went on to get a masters degree in a different field from a different school. I did well in both programs, including a few years of research during my undergrad. That was a few years ago, and since then I have been working professionally outside of academia. My goal has been to eventually go back and earn a PhD.

That is still my goal, and although I didn't behave unethically during my masters, I am troubled by academically dishonest actions I took as an undergrad and I'm not sure how to reconcile them. There are two specific incidents that particularly bother me.

In the first incident, I gave a class presentation that kind of followed a secondary source while only citing the primary sources. I now understand that is also plagiarism. In the second incident, I was panicked because I was struggling in a class. Another student had evidently found the solutions manual to the homework. He approached me, and I ended up giving him a small sum of cash in exchange. This is especially bad.

I have a few questions. Although it could certainly never happen, If I faced sanctions from the undergraduate university, would they revoke my degree? Would sanctions prompt the other universities to revoke my other degrees, including my masters and a PhD if I had earned it? How should I move forward? Would it be wrong for me to enter a PhD program?
Sounds like you have had some insights to your past behavior, and understand what you did was wrong - that could serve you will in future situations where you might be tempted to game the system. That experience could be a positive rather than a negative.

You escaped any consequences, and I don't think it's productive to think about hypothetical situations where you didn't, especially for something that is at least a few years in the past.

There are lots of good reasons to not enter a PhD program. I don't think you've found one here, but you should think about what goals you would have as a PhD student since a PhD is unlikely to advance your career unless you want to enter academia. You'll need a lot of internal motivation to get it done. Focus on those things rather than dwelling on your past indiscretions.