Feeding in from a similar personal experience - in this case PUBG rather than Warzone, but the principles will be the same - I'd go with the advice of others, stick with the 1440p 144Hz. I moved from running PUBG chugging along on an older laptop (860M GPU) @1080p to running on a desktop 2060 Super @1440p (think I have the same LG display as you). I'd say that the additional clarity of the 1440p display (especially for spotting people at long range), and removing the framerate drops, has been far more useful than the raw FPS increase.
You could try what JamiePhillips has said, I think the Series X is pretty open for keyboard & mouse compatibility (possibly as simple as plug-and-play) - although you'll probably be bad with them if you're used to controllers.
To be honest, spending a load of money to squeeze out a few FPS extra on a lower resolution for a specific game doesn't sound worthwhile to me, but I guess it's your money!
You could try what JamiePhillips has said, I think the Series X is pretty open for keyboard & mouse compatibility (possibly as simple as plug-and-play) - although you'll probably be bad with them if you're used to controllers.
To be honest, spending a load of money to squeeze out a few FPS extra on a lower resolution for a specific game doesn't sound worthwhile to me, but I guess it's your money!