Don't worry about it. Business is business. If the company was going underwater, do you think they would give a second thought to terminating you? If you do, you're naive and you need to snap out of it, because the answer is a thorough and resounding NO. And thus, you should treat them with the exact same amount of respect they treat you, which is none.
Here's what I got from your question: When Covid-19 happened, salaries were slashed. Do you know if salaries were slashed across the board? I presume you don't (such information would be hard to know), but given the fact that only junior employees (in the sense of people below management level) left the company, it's reasonable to assume that management was not affected by the salary cuts, only the low level employees were. Which tells you how much you, as a low-level employee, are valued. They could have chosen to not cut salaries, or they could have chosen to make across-the-board cuts, which would have seen more people from all levels of the company choose to leave, but instead they made all the lowest level people take the punishment of a decision they had no input or control over. If that's not enough to show you how much respect they have for you, then I don't know what is.
Now, you know you have talent. They also know you have talent. They know it would be a big cost to lose you. You have a competing offer that you said is once-in-a-lifetime. So you can continue working at a company who has already shown you that they live and die on management cronyism, or you can take this other offer from a company who respects your skills. But it looks like you've already seen and decided that part of it.
As for how to leave: When the company decided not to cut salaries for management, they did so by taking the stance that they believe management runs the business, not the junior employees. That's the official stance of the company. So my suggestion would be to have them put their money where their mouth is. Metaphorically speaking, you should tell them, "If you think you can do this job without me, then let's see if you actually can" (don't actually say that literally, but act in such a way to communicate that). It's literally the one and only job of management to ensure that if you leave the company, that the company doesn't die. If they are so highly respected that they didn't get the same Covid payroll slash as everyone else, let's see if they're worth it. If you leave and the company dies, then it was a shitty company with mismanaged priorities and horrible management and you jumped ship at a good time. If you leave and the company doesn't die, then everything is fine and this is a problem you didn't need to worry about in the first place. Either way, you win. So just leave. Don't apologize, don't "make amends", don't do anything beyond your normal duties, requirements, and responsibilities. Just leave and be done with it.