So do you vote third party?

If there's someone downticket worth supporting. Otherwise, just submit a spoiled ballot with nobody at the top.

Single-issue voters vote based on a single issue.

Its a big issue. But more importantly, there is no brilliant silver lining to vote for.

You're talking about a guy whose three biggest legislative achievements over his four years in office have been

• the Inflation Reduction Act - $783B in "spending" that was 85% tax-incentives (aka tax cuts) for already obscenely wealthy energy companies. The state of Texas has been the biggest beneficiary of these credits, incidentally.
• the enormous leap in security spending, both for domestic police (remember when we cared about BLM and defunding our bloated prison industrial complex? Biden doesn't) and border patrol (remember when we cared about Children In Cages under Trump? Biden doesn't) and our foreign aid to Ukraine, to Saudi Arabia and Jordan, to Pakistan, to Taiwan, to South Korea and the Philippines, and across Latin America (particularly Haiti). Even setting aside Gaza, all of this sucks ass. A repeat of Bush's Global War On Terror, right down to the "arming of moderate rebels" shit that always bites us in the ass in the end.
• "Ending" COVID, which appears to involve ratcheting back all the regulations and financial provisions for combating it, while the disease continues to do a 9/11 every two weeks in domestic mortality figures.

Oh yeah, and inflation. He's overseen an enormous spike in domestic prices, without any comparable rise in wages or public services. The last four years have been a series of empty promises and thread-worn excuses. This, from a guy who says "Bipartisan" every chance he gets, while bemoaning the threat of a resurgent rival party.

If they disagree with a party on 99.9% of issues but they’re the only one supporting their single-issue

Even Reagan drew the line at 80% and he was a hand-puppet for the party at large. This isn't about voting single issue. This is about witnessing a genocide on the watch of a guy who ran on being a civil rights champion.