Welcome.
A companion voucher is for BA metal only. From the USA you can only fly to London and onwards if you want to use a companion voucher.
Your main problem will be finding availability from LAX/SFO-LHR. You can add an onwards BA flight, eg Europe, Africa, Asia, etc.
Yes- that does seem to be the biggest hurdle. Bummer as I had hoped to book this year using First, but it looks like we may have to do it the following.
I went through the links - thanks. The Avio spending guide was great.