Well is is actually a 1980s house with plasterboard ceilings and yes I have very easy access so your point brings me full circle to what I originally thought about spreading the load – I was thinking I could coachbolt through a couple of joists and attach a cross member to take the weight of the chairs.
OH wanted one of those egg shaped chair things, but never got around to it.
Something to bear in mind is that it will swing around, so a long bolt up through a beam above across several joists will move quite a lot (ripping up the plasterboard).
My idea was to do that, but run the bolt up the side of a joist and firmly affix it to the side to take sideways loads. If the joist’s big enough you could drill straight up through it with steady enough hands?
OTOH, a screw straight into the joist will be more than upto the job. I have an eyelet into the ones in the garage for a suspension trainer held in by 4x 5x40mm wood screws which takes my weight (100kg), plus reaction forces from my feet on the wall, plus the fact that the load isn’t acting straight on the screws, plus dynamic loads. It must be taking 3000N easily.