(Sherrod) Brown doesn't really need to declare before his re-election campaign in 2018 and I would guess whether he chooses to run or not depends on whether he's re-elected and the choice to run for 2020 is not going to impact a re-election campaign, should he want one, in 2024.
But I also think you overestimate the risk it is for them to run even in the year they're up for re-election. In a recent example, Rubio essentially gave up his Senate seat to run, then literally gave up his Senate seat by announcing his retirement, then ambled into the election campaign at the latest moment possible and said "yeah I'm going to run again", and still easily carried the FL seat. Yeah, Trump carried FL and the coattails were good, but nothing about the 2010 -> 2016 transition suggests Rubio was impacted by his decision to run and that's a purple state. Running also does not appear to have hurt Kasich (Gov, also a purple state), or Paul (red state), or Sanders (blue state). You can make an argument it hurt Christie (blue state), I guess.