I know that you sell the Combi-Plan tank here in the US but I got one a number
of years ago and I found it to be junk. When filling the tank the fitting on the
top "bubbles" whatever solution being poured into the tank out of the tank with the
air this makes a mess when filling the tank. Trying to fill the tank to quickly
causes the chemistry to leak out of the air vent making an even bigger mess. This
leads to uneven development under the best of circumstances.

Of course I could simply have gotten a defective unit. :-)

I haven't tried to process more than three tanks at a time, but I've
done that with tanks doubled in length by adding a coupler and second
section of tube, to handle six films in three tanks.  My main objection
to the BTZS tubes is they're not very handy if you don't have a proper
darkroom -- I don't like the idea of handling the developer filled tube
cap inside my changing bag, and see problems with trying to keep the
tank in the upright, inverted position (i.e. film at top, developer at
bottom) while loading more than one tank -- not to mention the logistic
nightmare of changing from dev to stop inside the changing bag, and then
from stop to fixer.  I can load six sheets into three of my double
length tubes about as quickly as I can remove the sheets from the
holders -- I recently loaded six sheets from 70-year old plate holders,
with film sheaths, in not much over five minutes from zipping the bag to
all tubes confirmed closed and ready to unzip the bag.

With a minor modification, my tubes could be used like BTZS tubes; in
fact, I just thought of a way they could be used with minimal liquid
(perhaps as little as one ounce per 9x12 cm or 4x5 sheet) and preserve
the daylight fill capability: use a secondary tube that contains the
developer and attaches to the fill tube.  I'd load the tanks as I
usually do, and after all are loaded, invert the tanks onto the
developer tubes.  As time starts, inversion of the tank would cause
developer to run into the tank, and after ten seconds flow time the tube
could be laid on its side and agitated by rolling as is done with the
BTZS system.  The developer supply chamber could be removed at any time
after inversion, since it's now empty and the tank is light tight
without it; the level of liquid in the tube would be low enough not to
leak through the fill tube even without the usual inversion cap.  Stop,
fix, and wash would occur just as normal with daylight fill (since these
solutions are typically reused, there's little reason to economize them
as might be the case with one-shot developer).

However: I don't see the advantage of this other than to economize on
developer, and based reports I've read of uneven development with only a
few seconds pause in agitation of BTZS tubes, I still prefer my normal
method, in which the tank contains enough liquid to cover the film when
the tank stands on end with the filling tube up.  This allows inversion
agitation just as would be done with a roll film tank, at the cost of
requiring six ounces of liquid per 9x12 sheet in the standard tank
instead of one or two.  Installation of a core in the tank would easily
reduce the required liquid volume to two ounces, if the cost of
developer were high enough to consider this worthwhile (it's not, for
me, using HC-110, Caffenol, and Diafine -- HC-110 is cheap, especially
at high dilutions, Caffenol ridiculously cheap, and Diafine can be
reused without issues of consistency and so becomes amazingly cheap).