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I’ve got the cheapo Titan one, good tip on the magnet - it was getting through the adhesive that made the most mess previously. Thinking about it, I could chisel along the support beams rather than all over but having seen the work elsewhere in the room, I doubt the screws are in a nice even pattern!
Dramatic_Hammer-
There are some more specialised tools for grinding surfaces that include dust extraction. Festool do some. I've never used them due to the cost, I doubt you could make it pay for such a small area.
You could cut an inspection hole, find the joists and cut alongside them through the adhesive using a track saw with dust extraction (the blade would suffer though). You would then be left with a much easier job levering with a crow bar and spotting the screws, or use a multi tool with a carbide blade to cut through the screws between the board and the joist.
Airhead
It is just a gritty job. No need for a grinder though, there are very few interior diy jobs where an angle grinder is the right tool! SDS drill with chisel mode is the best thing for removing tiles. I had to remove lots of tiles by hand before I could afford one and now I treasure it.
Finding screw holes under tile adhesive is a bitch, some people recommend a neodymium magnet.