I don't think we can look at it that simply.
Of the voters who didn't vote for Trump or Harris, certainly some of them would have voted for Trump (or not voted at all) if their third-party candidate didn't exist. Put another way, if we had universal approval or ranked-choice voting, it's plausible that Trump would have ended up with a popular vote majority (but it could have gone either way, certainly).
> there were approximately 90M people who were eligible to vote that did not vote.*
Whenever an election is decided, I generally move all of those uncast votes over to the tally of whoever won. Staying home and not voting is the same thing as a soft-vote for whoever wins.
> voter suppression
I wish we could really quantify this. Not that it would change MAGA minds about anything, but it could be useful to know.
> a massive amount of ad money
Harris had a similarly massive amount of ad money, too, and it didn't help.
Honestly, I think the failures of the Democratic party machine as a whole are just as responsible for Harris's loss as GOP underhandedness is.