'Lay back' posts come from the BMX bike market, right?
Regular bikes have a 'set back' dimension .
That is from a plumb line thru the BB.. back ..
+1
I think you should start planning to replace the frame with the correct size.
if you need a big lay back and a tall seat post, then the frame is outgrown.
BMX is not Ridden seated, anyhow..
.. and now back to carbon fiber seatposts and their benefits to the OP.
and as i cannot see the bike's fit of the rider on the bike,
the 'should I get' I'd say , no way to tell.
get both , cheap ones ...and try the fit, then when you have it dialed
aside:
FWIW, Brompton parts, their Saddle adapter pin sits horizontal
on top of a plain type seat post .. then
there is their Pentaclip a saddle clip that is stepless,
and can be places ahead , behind, or on top of the SAP,
set back is an adjustmet of a few inches then..
after you have the dimensions taken .. Then go upmarket.
may need a non standard seat-tube angle , so off the peg Frame
may not even work..