Mach speed has a lot of hits in it. Generally, for me, if I hit mach-speed after an m-upper series, I can only land a j.c into TAC anything more and they’ll flip out. If you do it after a ground series (e.g. j.s, cr.a, cr.b, st.c, cr.c xx mach-speed), you can land Zoogstin’s finisher into Six Cannon, or OTG six Cannon without much trouble. Optimizations depend on your assists as you might be able to squeeze in more damage.
If you land the overhead, it’s always best to cancel into m.upper since you can do more damage from that than launcher. From straight launcher you’re probably limited to zoogstin’s finisher with more air-joe hits. (e.g. S, j.b, j.b, j.c, dj.b, dj.b, dj.c xx m.rhk, m.rhk, tj.c xx l.air joe (10 hits) xx six cannon (or j.s OTG Six Cannon)
Slow combo: whatever into m.upper, m.rhk, adf, m.rhk, h.rhk (land), j.c xx m.rhk, adf, m.rhk, h.rhk, (land) j.c, m.rhk, adf, h.rhk (land), from there you can launch and you’ve got as much HSD after a Mach speed after an m.upper series, so you can either launch into j.c, TAC, or do a delayed mach-speed into the same.
There’s also some possibilities with just slow, m.upper, m.rhk, h.rhk, but that might only work in x-factor and not slow.