Previa was running “ok” before tuneup? Of course you had low compression in 1 cyl before tuneup. But it could cruise at 70mph ok before? Odd that tire went out of balance during tuneup.
If the compression (which sucks) was known before the brake, shock, O2 work and so on was done the mechanic should stop, do not pass GO, do not collect 200 dollars, and advised you that sinking a lot of money into a sinking ship is not a good idea.