After reviewing the most recent article and all of the videos from Roaming Railfan, I'm pretty sure the slow items are:
(1) The bridges on the existing route where they are building a temporary bridge before replacing the bridge. Those temporary bridges are all started, but seem to be going pretty slowly. In several cases there are grade crossings which aren't being done until the bridge is done because the second track space is being used as the construction access road for the bridge; waiting to start the crossing work until the adjacent bridge is done adds to the final project completion time. Theoretically they should be able to begin service without these bridges being complete, but there would be rather long single-tracked sections which would not enable a full service schedule or full speeds. Financially, it still might make sense to introduce some service even if one or two of these bridges wasn't complete, if it started generating Orlando revenues earlier.
(2) The overpass of 528/Beachline over the new ROW east of the I-55 cloverleaf, which is nothing more than some piles of dirt at this point -- and involves coordination with the expressway authorities. This is critical and they can't operate a single train to Orlando without it. (They can't do a partial opening north of West Palm Beach without going all the way to Orlando because none of the intermediate stations between WPB and Orlando are anything more than design concepts at this point, so it's absolutely necessary to complete the track to Orlando in order to add any service.) This elevation/relocation of 528 is not scheduled to open until mid-August 2021.
All the bridges on the new Orlando-Cocoa ROW (and the second tunnel, and the trench) look to be well on schedule to complete in mid-2021, except for that expressway overpass. On the existing line, the grade crossings, double tracking, and signals everywhere except the bridges look like they're well on schedule to complete in mid-2021 too. The maintenance facility is supposed to be ready to receive trains (though not complete) by September 2021.
If things go well, they could be open around May 2021; the critical path appears to run through that 528 elevation, if I'm not mistaken.