New Border- Advice please

Well I have finally started planting up my border, my first Q is should I plant my Sarah Bernhardt Peony deeper by a couple of inches ? Please see the attached photo, some of the root is visible but also so are some tiny new leaves, I’m not sure if I need to cover it lightly with soil or not ? Also there are three small Acers, wasn’t planning to buy that small but was persuaded to buy my OH at Morrisons ( including the small chrysanthemums ) ! I will give them a chance, so far they look good. There is a patch on the left hand side at the back that is mostly shade and little sun, need to decide what should go in that area. I have also since planted a Mahonia Soft Caress and some Dahlia bulbs as well Any tips and advice would be appreciated, thanks in advance.
I have left gaps for some grasses and summer plants like Dahlias/ Gaura Lindhimeri, echinacea, Salvias

Ps the garden is east facing and most of the border gets a full afternoon of sun in the summer.

 

You've got a really nice big border on your hands there .
Yes you can add a bit more soil round the peony but don't cover the top of the root ( tuber ) they like a baking by the sun it helps them flower, you could probably go upto where the new shoots are coming through don't worry about the little one on the left .

I can see a few plants near the fence - euphorbia ( ascot ?) a lupin they are more middle border plants look a bit odd in that position . And split the Euphorbia up they can get quite large width ways but if that the effect you want like a mass of euphorbia keep them together . And the Hosta could do to be moved as well you'll not see it when the other plants go in , mid to front border plant they'll appreciate some shade but they do fine in the sun, they'd look nice under planting the Acers when they've done a bit of growing .

May need to move the lime green acer a bit further away from the fatsia it will get swamped .

Keep a eye out for the slugs they'll nip the tops off the dahlias

 

Thats all looking good for a nice summer display.

Seems you have to be very careful with the depth of the Peonys crown, think we would leave well alone as its sending up plenty of new shoots.
Planting tips - Planting peonies for beautiful blooms

For the Dahlias, the emerging soft shoots need slug/snail protection, we use a ring of copper tape around them and such similar tender plants.

Think some Hostas in the shadier parts might add to your planting theme ?

 

Thanks for the advice everyone. The border was already this size so we just altered the edge for a better shape, took out a few woody shrubs but left the Laurel. I intend to have a mini pond water feature in front of the Laurel as the soil isn’t very good there and to have a focal point. I plan on moving the Acers to give them more room and also five them a support stake and yes I plan on getting some either Hostas / Ferns and Japanese grasses for the shady spots. I will definitely leave the Peony as it is but I will move the Lupins.
The hardest for me part is knowing how much of a gap to leave between plants for growing room, never had a border or garden this big !