Our structure does largely lock in a duopoloy, especially at the Presidential level. You need a majority of Electoral Votes, so if a 3rd party started regularly getting a few dozen, we'd regularly have the election being sent to the House (which then has it's own rules as for how they pick a new President). You could go to pure popular vote with a requirement of say 10% margin or simple majority in the first round (e.g. if you won 45%-10%-25%, you're president, or if you get 50%+1), or you could require majority in the first round and then it goes to a top-two runoff. Some states, such as Georgia, already do that for Representative and Senate elections. You could couple new structures like that with ranked choice voting, like Maine now does, where you pick your first and second candidates. So your 3rd party, longshot vote isn't "wasted." They implemented this after the centrist and liberal dem candidates kept splitting about 60% of the vote, allowing the far right governor to win with 40%. It came into play in the 2018 elections.
If you want a multiparty democracy, you'd have to fundamentally reform our structure top to bottom I think. Here's a recent example of how a proportional representation parliamentary democracy might look like in America:
e: potential downsides of structures like this may include situations like Israel's most recent elections, where nobody could form a governing majority in parliament and they had to have 3 or 4 elections before a temporary COVID coalition was able to be formed.
There aren't too many Presidential republics compared to parliamentary democracies, and the structure of our particular one is unique given that it was created over two centuries ago.
In a multiparty parliamentary system, I wouldn't be forced to support Biden, and a number of conservatives wouldn't reluctantly be voting for Trump. Our coalitions and politics would look radically different, so it's hard to know exactly how things would shake out.