That is the speculation. Here is some well-informed speculation: Librem 5 v2 spec-ulation - #73 by amosbatto
If time-between-charges is the most important thing for you then “defer to Fir” is probably a good idea.
Bear in mind though that noone knows for sure when and with what specification Fir will come out.
Perhaps you can order now, deferring to Fir at checkout and then, if reports coming out of Evergreen (by then there should be many active users reporting their experiences, myself included) are positive enough for you, you could undefer i.e. go back into Evergreen.