I like a rock type move on d. snorlax myself when running thick fat since rock covers d.snorlax’s two weaknesses, bug and flying. However, I think that the best rock type moves that d.lax can learn, rockslide and stone edge, are post e4. Synthesis is great, just be aware that d.zard’s noctem will make it less effecive when night is up. I’ve played myself multiple times that way. I would note that you seem to be pretty weak to ground types defensively, half your team gets taken to town by earthquake. However, 2/3 of your team can hit ground types supereffectively so if you’re fast enough and hit hard enough it might not be an issue.
One thing I would do is put belly drum on d.lax. It makes it so that you ohko sweep every pokemon. After you belly drum, just heal up so that d.lax does not die.