General advice for starting out garden

Hi all, I've recently gotten into gardening and have decided to see what I can do in my rental properties garden as a fun project. It's a decked garden with an L shaped bed on two sides of the garden, it's not particularly deep which has caused me some trouble but so far this is what I have planted.

2 x Rhododendrons, the larger of which I just dug a space in the bed and put the entire plastic pot in it as it's far too big to plant in the bed.

2 x Azaleas, planted in the bed

3 x various small hydrangeas that I cut back last year (a little too much I fear)

1 x small Acer

I also have some climbing hydrangeas in large planters on the deck itself, along with some wisteria, jasmine, parthenocissus and pyracantha in planters that I have built around a pergola I built myself from raw timber.

To start out, I just have a few questions, if anyone can answer one or all of them, I'd be greatly appreciative.

1) Do I need to add anything to the soil for my hydrangeas, azaleas or rhododendrons? I don't know what the state of the soil is, are there any tests I should do to see how much nutrition is in it? Should I fertilise now they're planted?

2) The rhododendrons and azaleas wilted in the heat and lost most of their petals. I've removed the petals but the heads with the stems are still on the plant. Should I remove these too or leave them? Will they flower again this year?

3) How can I encourage my Pyracantha to grow sideways and become fuller/bushier rather than taller?

4) The parthenocissus lost most of its tendrils and leaves when I had to move house recently and had to rip it off the trellis. Its now growing leaves in clumps on a few of the remaining branches but I want to encourage it to grow more tendrils and branches/stalks to grow new growth and thicken it out. Is there any way to do this?

5) My Trachelospermum Jasmine was also distressed from being pot bound for a while and the move, its now nowhere near as thick as before and feels a bit brittle, though it's not dying. The leaves have mostly gone back to green and the brittleness is starting to fade but it's only growing new tendrils very slowly and there are no sign of flowering buds yet. What should I do to nurse this back to health?

6) Lastly, in the bed around the hydrangeas, azaleas and rhododendrons I'd like to fill out the ground cover and in between spaces with things that won't take away from those shrubs but will provide good seasonal interest, flowering at different times and plenty of colour/texture contrast. Can anyone suggest good plants for that?

Thanks again for any help!