specify a border and exact space between images

I want to have a picture like this:

TTTTTTTTTTTTTTTT
E P1 F P2 F P3 E
================
E P4 F p5 F P6 E
BBBBBBBBBBBBBBBB

where P{1..6} are rectangular pictures which are equally sized, E denotes some horizontal spacing, = denotes some vertical spacing, T denotes some top spacing, and B denotes some bottom spacing.

I would like to do this with just one command, but I am fairly sure that it's actually not possible to express this with montage.

It is probably possible to do it with multiple montage calls, however. I didn't try that yet, but more in general it seems montage is a rather special purpose tool with a rather weak grammar to express pictures.

Is there something better for this particular task (scriptable)?

If T, B, E, =, F are different values, and P{1..6} are different images. It might not be worth attempting short-cuts, but build out each row, and column specifically. The following example is one ImageMagick command using bash variables as placeholders.

B="-size x15 xc:"
E="-size 5x  xc:"
F="-size 10x xc:"
T="-size x25 xc:"
V="-size x20 xc:" # V is an alias for `=`

EQUALLY_SIZED="-size 20x30"
P1="${EQUALLY_SIZED} xc:CadetBlue1"
P2="${EQUALLY_SIZED} xc:firebrick"
P3="${EQUALLY_SIZED} xc:ForestGreen"
P4="${EQUALLY_SIZED} xc:lavender"
P5="${EQUALLY_SIZED} xc:LemonChiffon"
P6="${EQUALLY_SIZED} xc:OldLace"

convert $T \
        \( $E $P1 $F $P2 $F $P3 $E +append \) \
        $V \
        \( $E $P4 $F $P5 $F $P6 $E +append \) \
        $B \
        -append \
        output.png

This example works because B, T, & V have no value except height. And E & F have no value but width. If we update the FPO spacer images as orange canvas...

O="xc:orange"
B="-size x15 ${O}"
E="-size 5x  ${O}"
F="-size 10x ${O}"
T="-size x25 ${O}"
V="-size x20 ${O}" # V alias `=`

... we can verify the expected results.