One mast, no duplex box, two meter-mains, no problem
The primary problem with your proposal as it stands in your post is that NEC 230.7 prohibits service and non-service conductors from being commingled in the same raceway or cable:
230.7 Other Conductors in Raceway or Cable. Conductors
other than service conductors shall not be installed in the same
service raceway or service cable in which the service conductors
As a result of this, you'll need to provide a service disconnect at the meter for the EV feed. The simplest way to do this is to make it a meter-main as well; although one could use a separate breaker enclosure for that, that would just take up space and create more issues with service entrance bonding and grounding. However, since you don't want to use a "duplex box" type multi-meter packaged assembly, this means we'll need two separate meter mains, and a way to make the tap connecting the two. Normally, Xcel does this via separate service masts fed from the same service drop using overhead taps; however, we can use slightly fatter conductors to accomplish this with a single mast and the correct metering parts.
In particular, since your utility accepts Milbank metering hardware, we can assemble things so that the service-entrance conductors feed the house meter-main, then are tapped, either now or at a future time, to feed the EV meter-main from below, as if it were an underground service, via a conduit off the bottom left-hand side of the house meter-main, where it ties into the house meter-main's utility-side underground service gutter.
This means you'll need a Milbank U5871-XL-200-5T9 for your meter-main instead of the Eaton MBX816B200BTS your electrician originally suggested, with a set of Milbank K4977-EXT tap lugs slid into the meter-main's line positions and an A7517 2" hub. This gives us the ability to accept SECs up to 350kcmil, with a maximum of 1/0 for the service tap to the EV meter. It also provides 8 breaker spaces that can use basically any 1" breaker you can find on the shelf, and a set of 200A feed-through lugs for the feed out to the house distribution panel.
With this, we use a Siemens MM0202L1200JLX for the EV meter-main, with a field-fitted Q280H for the main breaker. This is then fed from below using a 1.25" LFNC from the bottom right corner of the house meter main, curving upward to go into the meter compartment of the EV meter main from below while passing over the 1" PVC riser from the T-body into the EV meter main. Furthermore, we can fit the tap lugs for the EV meter now, but leave the EV meter-main and service conduit unmounted for the time being.
Physically speaking, this puts the EV meter-main on the left and the house meter-main on the right, mounted so that the EV meter-main's customer-side bottom KO aligns with the existing 1" PVC and the meters are at the same height, somewhere between 4' and 6' off the ground to the center of the meter. A 2" RMC mast with 350kcmil Al XHHW-2 wires in it is used for the main service-entrance conductors, going down directly into the top of the house meter-main via its hub fitting From there, 4AWG copper THHN wires (two hots and a neutral) are run down through the house meter-main's underground service gutter and through the 1.25" LFNC for the service tap from the house meter main to the EV meter main. Finally, the grounding electrode conductor lands in the house meter-main, as depicted in your picture.
Sidebar: if you really want to use somebody else's meter main
If your electrician is dead-set on using the Eaton MBX816B200BTS, the concept proposed above isn't completely nullified, thankfully. Provided the utility is OK with it, one can use a pair of insulation piercing tap connectors (Ilsco IPC-350-4/0 or equivalent) to tap the hot conductors in the meter-main, then use the utility-side grounding lug on the meter-main to land the tap neutral.
Now that we are properly served...
Now that we have the service-entrance hardware sorted out, we can tackle the feeder conduit to the garage. The existing conduit gets routed through a T body and some more 1" Schedule 80 PVC up to the EV meter main, past the right side of the house meter main. You'll need a set of raintight reducing washers (Bridgeport 1073-RT or equivalent) to go from the 1.5" or 2" provided by the knockout on the bottom right of the EV meter main to the 1" of your conduit, in addition to the obligatory PVC male adapter, by the way.
With that out of the way, we move onward to the garage end, where a 100A, two pole, NEMA 1 (indoor), unfused safety switch gets fitted next to the existing panel, connected by a 1" rigid nipple. This will provide the required disconnecting means for the EV charger branch circuit, while the panel main breaker does the same for the feeder.
Once this is done, we remove the old wires from the conduit between the house and the garage, unwiring the outdoor light switch in the house at where it ties into the conduit run. We then ensure that the bonding screw in the garage subpanel has been removed at this point, in addition to making sure that the garage has its own ground rods, connected to the grounding bus at the garage subpanel with a minimum of 8AWG copper.
With that verified, we can pull the new wiring: two 4AWG copper THHNs for the EV charger, three 10AWG THHNs for the feeder to the garage subpanel, three 14AWG THHNs for the floodlight switch, and an 8AWG bare copper ground wire for all of this. The 10AWG and 14AWG wires get pulled through to their existing destination inside the house (where they transition out of the conduit, or into boxes), and then connect to the garage panel and garage floodlight circuit, the same way the existing feeder and floodlight wires were connected. The 4AWG copper THHNs and the bare 8AWG copper grounding wire terminate in the EV meter-main at the house, end, then, with the grounding wire landing in the grounding bar of the garage subpanel and the two 4AWG hot wires getting pulled through the garage panel and the nipple to reach the line lugs on the safety switch.
Last but not least, the existing grounding wire from the panel to the conduit entry point is connected to an insulated 10AWG grounding conductor run from the EV meter main, through the conduit body, to the conduit entry point inside the house. This is done this way to avoid sending any current on the EV meter-main's neutral out to the garage and back as objectionable grounding current or splicing in the new T conduit body (which may cause serious box fill problems even if the body is marked with its volume and thus legal to use as a housing for splices). Note that if objectionable grounding current does become a problem here, you are permitted by NEC 250.6 to disconnect and remove this insulated 10AWG wire and thus break the parallel path between the two bonded meter-main enclosures.