I would pull the 200A conductors now so that you don't have to buy another set later
What I would do in your situation is pull a set of 4/0 Al XHHW-2 conductors through the conduit with a 6AWG bare copper ground (these are sufficient for a 200A feeder to a dwelling unit), provided I was confident the service move was to take place. (If you really aren't confident about the move, you can pull 1/0 Al XHHW-2 now along with the 6AWG bare copper ground and put the 4/0 conductors in later -- the price difference is a bit under 40 cents to the foot per conductor.)
As to making the tap, for now...
I'd use a set of insulated mechanical connectors (often called "Polaris connectors" in the trade) and aluminum pigtails (short lengths of 1/0 will do) in the existing meter-main box to make the tap for now -- this is the simplest way to do that.
And what's going on in the garage
At the other end of this feeder branch (it's not a "tap" technically under NEC rules because it's sized for the full 100A current), I would install a full size subpanel in the garage -- a 30 space, 100 or 125A, main breaker subpanel will do the trick. This way, you only have to buy one subpanel for the garage instead of buying a small one now and a big one later, and you'll also be able to keep the existing breaker and wiring for the EV charger.
Last but not least -- meter-main suggestions
My personal choice for a meter-main for your proposed upgrade, given the large feeder to the detached garage and the need to use an EUSERC-compliant ring-type meter mounting system to meet PG&E requirements, would be a GE TSF420CSCU "farm panel", with the garage breaker (a TH(H)QL2100) in the upper (secondary) disconnect spot and the house panel fed from the main section's feed-through lugs. This way, the garage and house have separate "main disconnects" if you will, while providing spare space for outdoor circuits associated with the house.