File multiple flight plans if departure time uncertain?

Yes, approach control or a tower sees the flight plan, distribution is from the center they belong to that handles their flight plans. FSS does not get a copy of an IFR flight plan if you file through ForeFlight, so are unable to amend a filed flight plan. If you were to specifically request an FSS specialist to relay a departure update to center, even though they did not have a copy of the flight plan, they would most likely do this on a land line for you. But otherwise, the specialist is going to respond they can't amend your IFR flight plan because they don't have access. There is no mechanism for them to call up your flight plan. I just called and verified this with an FSS specialist.
Do you know if other Apps, like Garmin Pilot etc. do it this way to?
Garmin Pilot offers filing through Leidos as an option or filing via I don't know how handles fling IFR flight plans.
Not my experience. Maybe they did call the center for amendment.

But I am pretty sure I told them the tail number, proposed departure time, departure and arrival airports and they called it up and said they would amend it, and it was.
I have filed multiple IFR plans before. A few times because I had the wrong tail number or other wrong information. Basically I abused the process. If I made a mistake instead of calling to amend, I just filed a new plan. Only once did a controller voice an issue and asked me which plan I wanted to fly. But that was when I was new to filing with EFBs, made plenty rookie mistakes.