They have two opportunities, agreed - it's a matter of apportionment of the marketing spend/intensity over the next 3-year period.
For sure, there will be the normal faire marketing and promotion for Pantheon - they likely want to nail the opening date, before pushing out any substantial marketing - I'd venture to guess that's why the marketing has been insubstantial to date. I wouldn't market anything beyond teasers, until it's cleared all the testing, i.e., the risk of eggus-omelette on everyone's face.
When I say 'big' with the marketing push, I mean at a scale that SEAS/BGW has not normally operated. While it may be a yawn for coaster enthusiasts, 2021 is the deliverable that puts BGW on the national map with the general public. For that reason, I could see the intensity of the marketing building to a 2021 crescendo. This overall marketing push needs to sustain BGW for a few years; I'd hazard to guess, as perhaps others have, the next significant step-up for the park wont come before the 2025/2026 period, coinciding with the 250th anniversary of the US.