I personally am not sure I would call anything inside a church building a sepulchre. This may just be my association, but I tend to associate that word only with the Bible story of the resurrection, the tomb in the rock; or at least, maybe something in a Mediterranean country, with raw stone featuring prominently in the architecture, like, say, limestone, and dating back more to, say, the Crusades than the 16th century. It would sound odder to me for some reason in Renaissance northern Europe; but again, maybe that's just me, perhaps others will comment.
I would be even less inclined to use *sepulchral monument, it just doesn't sound very idiomatic.
Even if it's fairly large, I wonder if just tomb wouldn't be better. Tombs are usually more freestanding, anything from stone coffins (though we wouldn't usually call them that) to things like mini-buildings. If it were the kind that's under the floor of a church, it could be a grave, but that doesn't sound big enough for yours.
Monument to me sounds quite large and more typically outdoors, like a large statue, pillar, or column; think of the Washington Monument in DC, or what's the big black one in Edinburgh? Inside a church, you could use it for a really massive and imposing structure, like a very large tomb, but in that case I might just stick with that word alone, without any other adjective.
Cenotaph might be similar, but many people probably wouldn't recognize the word. I would be hard put to define it myself; all I know is that I would associate it vaguely with classical antiquity (Greece, Rome, etc. ...).
Tombs in churches do sometimes have effigies on them, especially of knights and such, but yours might be even bigger than that.
It does sound bigger than just a memorial tablet in the wall.
I've never heard of hatchments in this context, or ledger stones at all.