Hi confused in KY,
The mold sizes they use are more a matter of economics. A twin XL can be fabricated into almost any size using just two pieces (the exception is a cal king which requires 3 pieces) so they can pour one size and then fabricate it into any other size they want. There are a few queen size molds which produce finished queen size layers without a glue seam but they are much less common because they there are fewer molds of this size (and they can only be used to make queen size or too much material would be wasted).
In other words … most of the “finished layers” for talalay latex (outside of twin and the occasional queen) would have a glue seam although you wouldn’t feel it.
Yes … as you mentioned you would be looking at the additional cost of a new bedframe as well if you changed sizes since your old one wouldn’t work although if you wanted to eliminate the sleigh bed (or sell it) there are some fairly inexpensive options available in the foundation thread that can be used as a base for a King size mattress.
Phoenix