The previous owner of the house expanded & remodel back in mid-2000, so I wasn't sure what wires they put in or the reason for 30 amp duplex breaker in sub panel.I'm confused. There should be no need to replace any breakers or replace wire running across the house if you keep within the 30 amp budget at the subpanel. I checked the subpanel and I >still< can't figure out why they'd put a 30 amp duplex breaker in there, presumably backfeeding the panel.
If the wire running from the main to the sub can accommodate 40 or 50 amps, you could get the breaker in the main panel upgraded and properly connect the subpanel(to the lugs meant for this very purpose), and then repurpose the now-useless 30 amp duplex breaker(or at least its breaker-spots) in the subpanel for your HPWC(if you want).
I guess the few quotes I got Tesla referred electricians probably want a bigger job and thus recommending to go full 40 or 50 amps charging, and they only way to accomplish that is do draw power directly from main panel (with new wiring etc.)