investing it as a lump sum on January 1st will produce higher returns more often that investing on a monthly/weekly basis

Correct, you're referring to investing via lump sum versus DCA (dollar-cost averaging). And you also need to consider your investment's time in the market - Most often the longer you are invested the better your returns are over a long timeframe. And this is where lump sum comes in since you are investing everything as soon as possible in the tax year.

But of course no one knows the future. A terrible year in the market = your early year lump sum looking like a bad bet. But we're talking about an IRA here, the whole idea is to leave it invested long term so even a terrible year should pan out on its own over time.

All that aside if you're really worried about it and/or strongly feel that markets are going be down for the entirely of 2024 then there's nothing wrong with going DCA (investing every week/month/whatever).