Id bet he been under a 320 OBP at least half the months here, probably quite a few under 300, and bet he probably had maybe 6 or 7 decent hitting months of at least 260 to 270 or better since he been here....
The way your predictions work out I'm even more convinced he'll get better.
As for the rest, he's literally been over .260/.320 the past 2 seasons. He hit .259/.326/.389 in 2017 and .270/.335/.395 in 2018. You're also fundamentally misunderstanding why he was "bad." To illustrate, do you know what Javy's OBP was last year? It was .326. So in other words, in 2017 Heyward's OBP was identical to the 2018 runner up for NL and and he was 0.9% better than Javy in 2018.
The reason Javy is substantially better is because of his ISO(SLG-AVG or extra base hits). Javy's was .264. Heyward's has been .094, .130 and .125 the past 3 years. That specifically is what killed him as a hitter in 2017 and 2018. 2016 is a longer story and i mean we're better off not talking about it. Heyward's ISO in 2019 is still sitting at .170. Take your pick of Heyward's best hitting season. It's going to be either 2010, 2012, 2013 or 2015. His ISO those years was .179, .210, .173 and .146. So, he's hitting for substantially more power than 2015 and within range of 2010/13. The .210 in 2012 looks a bit of an outlier and I'm doubtful Heyward ever hits for that much power again but the point here is his best seasons have all come when he's been up over .140 ISO.
Additionally, I'm not sure why you're even mentioning his OBP negatively. His 13.3% walk rate is the second best in his career behind 2010 at 14.6%. As I told you before, this all boils down to a comically unsustainable May BABIP. I mean it's 114 points lower than his 2016 BABIP which was 11th worst in the majors that year. And before people come in here saying he was just lucky in April, his April BABIP was .313. So he wasn't even that fortunate. That's all of 16 points higher than 2018 and when you figure how much harder he was hitting the ball... that's really not that surprising.
And all this is beside the point because the argument you're making that he's a "bad" hitter just isn't accurate. He's an over paid hitter specifically the last 3 years certainly. But an 88 wRC+ in 2017 and a 99 in 2018 really isn't that bad if you take the money out of things. Jackie Bradley Jr. hit .234/.314/.403(90 wRC+) playing 144 games for the World Series winning Red Sox. The money at this point is a sunk cost. So even if you don't think he's any better than 2017/18, you can still win a world series with a hitter like him playing 144 games. And clearly I think he's better than that this year.