Whatever you choose to plant is going to need some soil improvement before hand.
Conifers really do remove all the goodness from soil, so you would need to add plenty of compost etc.

Might save you some years of waiting if you remove as many roots as you easily can rather than waiting for them all to rot down.
Alpine like lots of sun but there are other plants that are less fussy' Some ferns will be happy in dry shade and would look nice. You could replace some of the needles with older compost and compost those separately to speed things along. Get yourself a pH meter before you decide what to plant. Those needles will make good compost eventually.
Be prepared for some hard work if you want to put up a trellis where those roots are!
You might sow some foxgloves now,hoping they will bloom next year to tide you over until you are clear where you want to go when the trellis is in.. If you are impatient for flowers I'd plant up some pots to cheer you up during the transition. Don't think of adding any small plants until the major work is done.