nice team man
have you ever considered skarmory > metagross? spikes fit in really well on this team, since you've a spinblocker and a phazer, not to mention how much they assist mixgon in breaking down stall. skarm has really similar typing and functions similarly to metagross in that it shuts down sleep leads with lum berry. by making this change, you lose two things: 1. metagross's offensive presence and 2. a stealth rocker. so why make this change?
first off, skarm's defensive capabilities fit this team more since it's focused on bulk and longevity, the latter of which meta doesn't really have since it blows up a lot. second, you've got another great candidate for setting up sr in sdef tran. you can drop earth power to make room, you don't have to hit other tran since you've starmie and flygon. i also recommend a full sdef ev spread for maximum tanking ability, and possibly protect over toxic, to rack up lefties recovery and lava plume burn damage.
back to the original suggestion, skarm: if you're interested in trying it out, the set is careful natured with the aforementioned lum, max hp / spdef, spikes | taunt | roost | brave bird.
gl, hope this helped