Metasequoia are adaptable, while growing out young trees, in large pots to thicken trunks, and get them ready to become pre-bonsai, any potting soil will work. I'm using a peat based soil for my young'uns. They really like water. They can be placed in a tray of water similar to how bald cypress are grown, though I use a more shallow tray for water than some use for bald cypress. The water tray is for summer. I removed it for autumn and winter.
For Dawn redwood in bonsai pots any of the standard bonsai mixes will be fine. They do like mildly acidic potting media, but are not fussy about it. Akadama-pumice-Lava mix is fine. All Akadama would be fine. A Douglas fir bark with pumice mix will work. Most of the tried and proven media will work.
they do need large amounts of water, daily. Whatever mix you use should hold water.