Hi OP
10m is a lot to fill! Not cheap either.
Look for plants online and see if you like them/if they do what you need them to/if they will survive where you want to plant them/if they will be too tall/wide too quickly, etc. Search terms like: perennials for shade/evergreens for shade/ and see what comes up that you like.
A garden writer (can’t remember his name except Larry, American) wrote a good book about shade gardening that I borrowed from the library because my garden is pretty much all shade except 1 part for a few hours a day in summer. He said to try the petunia test! That is, plant a petunia (a bedding plant for late spring/summer) and if it doesn’t thrive, you know that you’re set for shade gardening. So, that’s something you could start with when they are in the garden centres! You only need one and it could save a lot of money.
Plants I use in my garden: a framework of evergreens so that there is always something green even in winter. This includes euonymus (variegated, green and gold and white and green, different varieties for different heights - the coloured leaf helps to lighten the garden and gives more interest than just green; Sarcoccoca (2 varieties, 1 taller (Purple Stem) 1 shorter (Hookeriana). Both have tiny white scented flowers in winter. Also a Laurel that I may have to get rid of because the pruning is a lot of work to keep it to a reasonable height - I didn’t plant this one); Mahonia “Soft Caress” - winter flowers again.
I have some different spring bulbs because there is much less shade from the trees while they are dormant so I can get away with some.
I also have Bergenia (spring flowering) which is also called Elephants Ears because that describes the leaves. That’s a low growing one, about 10” and makes a bigger clump every year. If it outgrows it’s space you can break bits off and plant elsewhere.
I also manage to get Erysimum Bowles Mauve to grow in my “sunspot” which has a greeny-grey leaf and purple flowers - the bees love it.
Recommendations from Larry-whatever for flowering shade plants for borders
included Tiarella/Heucharella, Lamium (White Nancy is a good one to brighten the garden, but there’s a pink too), Brunnera, Pulmonaria, some of the hardy geraniums (Cranesbills) like Rozanne,
Aquilegia, Dicentra (Bleeding Heart), Hellebores and Astrantia. There will be more that I have forgotten.
If it turns out that you have the kind of shaded garden where you need plants which will thrive without half a day’s sun, I hope some of these will help get you started.
Enjoy!