Creating a hosta bed

First of all you are making a mistake with pachysandra it will now take over the hosta.Seondly they take time to grow. third you never mentioned you location>>>

Each year the hostas will be bigger and fuller, so don't plant too much around them, there is a chance that nothing will grow within them once the fill out, unless you planted them too far apart. They will get thick. I have some growing along my sidewalk and even weeds don't have a chance under them. Any heat tolerant, shade tolerant plants will be great behind them to add height to the plantings. Most of my front yard only gets a short time of early morning and late evening sun, so not much more than hostas will grow there. The only other things I have had luck with is tuberous begonias in pots and elephant ears and coleus in my huge half barrel pots because every thing else gets leggy from total lack of light.

I understand your problem I have the same thing going on and I really don't want plants that flower in shade but if you do how about hydrangea that are for the amount of shade and heat you get? I plan on adding ferns. I have coral bells and they are also taking forever but it hasn't been that long so I figure this gardening thing is like watching kids grow sometimes they just appear to take forever to mature! I personally have given up on annuals it's such a waste of money except for impatience they really fill out areas beautifully and aren't that expensive and don't attract bees and won't grow taller than what you already have. Those I like and I break my rule on but I plant a lot of those and it is back breaking. But hey until my impatiences fill out I need to be patient.

Hostas require a lot of water and less heat so that is probably why they are slow growing. They sound like they are stressed. My hostas grow at least 3-4 feet wide and four feet high but I am in a cooler area. Sorry, cannot recommend other plants for your area