In case of standups, if the developers aren't available then what's the point of having the meeting? Since DSUs are for the Scrum Team to update each other about the daily progress and to plan for the current day, its mandatory for the team members to attend it. If the developers aren't available at particular time of day, then the Scrum Master could get their updates from them via face to face/phone conversation/chat/email prior to the scheduled DSU but if the developers wont be able to attend the DSU on regular basis (due to a conflict) then this meeting needs to be rescheduled. Ideally DSU's should be scheduled in the morning session.
DSU's need to be scheduled on a set time and at the same place on recurring basis, (in COVID days place doesn't matter) and shouldn't be pushed/rescheduled on daily basis.
In regards to the generic project meeting, I don't know the purpose, attendees of that meeting but generally project meetings have key stakeholders which have busy schedules and they may not be available some other time so that could be the reason for not rescheduling this meeting.
