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These can benefit from some light pruning and deadheading, to control their shape and size. It could also be that they are lacking light if they are in a shaded area, or if an area seems to get too dry.

Here is an article to help you with their care: https://www.gardeningknowhow.com/edible/herbs/hyssop/cutting-back-anise-hyssop.htm

This depends on where you are located. The plants are hardy in zones 4 through 10. Outside of these zones it will need some extra care. Within these bounds, the plants will do just fine on their own. Here is an article for more information on the care of these plants: https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-pruning-info.htm

Generally, a five to 8 gallon container will suffice. This will give it plenty of room to grow, while not being overbearing to move if necessary.

You will probably get a hybrid. In order to obtain true seeds, there must not be any insects, or bees to pollinate with any other variety that is around.

Usually, this can be done indoors, where the only plant in the area will be your target species

You can still plant your seeds, and expect a plant that has its own, unique look to it, though.

This article will offer more information on the plant: https://www.gardeningknowhow.com/edible/herbs/hyssop/growing-agastache.htm

You could get a perennial agastache in that color. But you might have hummingbirds and butterflies trying to land through the glass. Something like a tall cacti/succulent would look nice.

These articles should help:

https://www.gardeningknowhow.com/ornamental/flowers/fgen/full-sun-plants.htm

https://www.gardeningknowhow.com/special/xeriscape/desert-plants-for-full-sun.htm