Seating Plans and the Family Politics

The plan takes an evening. The negotiation around it takes three weeks and causes more arguments than the budget.
Do It Late and Do It Once
Do not start until the RSVPs are genuinely closed, because every late change cascades. Then do the whole thing in one sitting with the people who need a say in the room, rather than over six weeks of messages.
Physical cards on a table beat any app. Everyone can see the shape at once, moving somebody is instant, and the argument gets resolved standing up in half an hour.
Long Tables Solve Half of It
Rounds force you to make eight-person decisions, and eight-person decisions are where the trouble is. Long tables mean you are only deciding who somebody sits beside and opposite, which is a far smaller problem, and people talk to more of the room.
They also suit a barn or a marquee better, need fewer flowers, and cost less in linen.
The Hard Cases
Divorced parents get separated by distance rather than by anything that looks like a statement, ideally with a table of allies between them. Nobody who has been asked to sit somewhere specific should find out on the day.
And put the people who will actually get up and dance nearest the floor. An empty dance floor at nine o'clock is usually a seating problem rather than a music problem.