I'm hoping that the considerable expense of dosing Excel regularly at fairly
high rates will help with the CO2 level, and that increasing the KH will
make the CO2 more available to the plants. I'm not looking for spectacular
plant growth, just a decent looking display that will enhance the tank and
make life more enjoyable for the fish. Thanks for your contribution to my
education.

Excel sounds like the deal for you. As long as your light is lower,
you should be fine.

Adding KH, -HCO3 in this case can help plants grow if they are CO2
limited, a few plants can use thie HCO3, most all algae can as a rule.

Adding it will not increase or lower the CO2 level. That will be the
same no matter what the KH.

Try this, add baking soda to a glass of water and measure the pH/KH do
the same for a glass of water without adding the baking soda. The CO2
levels should be identical.

The KH will be different, the total carbon will be higher in the one
with bakign soda added, but the CO2 is still the same.

If you want to increase the CO2 level, add CO2 gas only.
That's what the plant need/want. We use pH/KH to measure CO2, not
control it's level.

CO2 gas is cheap, the gas tank systerms run about 125-150$, the DIY
are good for small tanks and are almost free.

Regards,
Tom Barr