Subfloor repair for LVP installation
Professional flooring installers would flood from the high point to the low point with self level.  They typically charge per bag and I've seen this kind of job cost 15k when done for more than one level of a house.  You can do as you suggest a hybrid approach.  Installers won't typically go that route because it is a lot more labor and requires a lot more expertise and judgement calls.  They want a fool proof and easy way to get the floor flat and there is more room to go wrong when making judgement calls.
Note you are going for flat not level.  Typically your finished floor material will say 1/8&quot; over 10' or something like that.  To see how bad your floor is you take a 10' straight edge and move it around the room, then I sometimes use horseshoe shims (these come in 1/16 - 1&quot; thick) and see which size will fit under any gaps that are visible under the straight edge.
Some things to keep in mind.  You can go outside the 1/8&quot; over 10' if you can tolerate being able to detect bounce in your floor - I am assuming you are going for floating install.  It is worth also thinking about what section of the floors are being used for.  You'll notice out of flat more if this is a walking area than if this is under furniture.
I've done the hybrid method before.  If you put plywood down first to build up the 1&quot; and then try to use SLC to feather out to the plywood you end up having a join between the edge of the plywood and the SLC and this edge is likely to crack.  If you just pour the SLC then there is no sharp edge transition.  You can mitigate the transition with different techniques.  This is why SLC is typically the one stop solution.
I hadn't heard of Henry 549 - it is interesting that it is a cement based SLC but doesn't require a primer over plywood.  Mapei and Sika both require a primer.
If you are doing the work yourself at $20-30 a bag you probably aren't much ahead using the plywood - labor is probably worse for the plywood with the cutting screwing and gluing but if you aren't investing in the SLC tools (spiked roller, squeegee, trowels, spike shoes, etc) then might be easier if you already have the carpentry tools.
If you are going the plywood layer method then filling in the large dip with SLC first makes a lot of sense.  The issue you run into is how do you adhere the plywood over the SLC.  I've screwed through it before and it can crack.  If it cracks and the pieces are able to move around then you can end up with noise when you walk on it.  Probably the best approach is to use construction adhesive on that section but you'll need to put a substantial weight on the plywood to ensure it pushed out any air.  Construction adhesive also can tend to be thick so it can end up pushing the plywood out of level especially if you aren't screwing it down (this is harder the thicker the plywood is.  You could trowel the adhesive onto the plywood much the way tiles are trowels do the ridges collapse and the air is able to escape.   Probably better to go with a glue used for glue down installs and use that to bond the plywood to the SLC over the area.  Hopefully the dip isn't huge - hard to tell how large the area of the dip is.
I'd expect any settlement to be finished.  If your joists are undersized and you have crazy amounts of movement in the floor it is possible the SLC would delaminate from the plywood overtime.  If you are installing with an underlay then I wouldn't expect a problem if it cracks but the floor is flat then any cracked piece won't have anywhere to move.  The underlay would further suppress noise of it between the finished floor and either the intact SLC or the subfloor.  Professional floor installers use SLC on super high end homes, if there were a tendency for it to breakdown overtime they wouldn't use it or there would be crazy amounts of lawsuits.  The henry 549 has a max lift of 1/2&quot; so you'll need two lifts.  The other thing to watch is that this product doesn't require a primer and the plywood is going to suck the moisture out of it.  Sika and Mapei require primers (on plywood it is a latex primer) which prevent moisture from being sucked out of the mix.  I'd go for cold water for extra work time and you probably want to prewet your plywood.   Both mapei and sika have SLCs that can be feathered.