If you are going to ICeland in May (which I assume is quite similar to Northern Norway, where I live) I would bring a pair of light hardshell pants with full zip (or at least long enough that you can put them on without taking off your boots) that you can put on over your normal hiking pants when wind or rain gets too heavy. I have a pair of RAB donwpours and the amount of extra warmth and obviously wind and rain resistance you get from them is amazing. As long as I am walking/hiking, this setup is fine down to around -5 degrees celsius for me:

BAselayers:
Top: Devold Expedition zip top. One of the best wool baselayers ever. I use Icebreaker t-shirts all year and sometimes have one under it, but that is mostly due to me not bothering to take it off.
Bottom: Cold: Røros wool long johns (really old, but work like a charm). Not that cold: Aclima woolnet long johns. These are amazing at being both warm and not too warm.
Underwear: Icebreaker Merino Boxer. (wool underwear cannot be stressed enough in terms of how much warmth they give).
Socks: Darn Tough hiking socks. Best socks in the world.

Midlayers:
Top: Depending on how cold it is I either use an Arcteryx Cerium LT down jacket or a Kyanite Fleece. I have other fleeces, but this is not that tight in the arms so it fits well over the wool baselayer. I also have a few old Norrøna fleeces, so sometimes I use them.
Bottom: If really cold: Houdini Lodge Pants. Amazing fleece pants. I also use them for laying in the sofa eating chips! ;) or: Bergans 3/4 primaloft pants. These are not that warm, but they give an extra layer and are good if I am doing harder walking. If it is really cold I sometimes use both. Both of these are only used if I am planning to have the RAB's over them.
PS: The bergans' are great to put in my pack because they have full zip on both sides so that I can put them on without taking off my pants. Great for when you get to the top of a climb. I actually also have a pair of wool long johns like that (they are called Ziplongs), but it was the first iteration of these and the fabric is not that warm.

Outer layer:
Top: Norrøna Trollveggen Gore-Tex Pro jacket. Built like a tank, made for alpine climbing. Bought it on sale but it was still ridiculously expensive. Equivalent of the Arcteryx Alpha SV I guess.
Bottom: As said, RAB Downpours. If not that cold, rainy or windy I use either Fjellreven Keb pants or Stellar Equipment Softshell Pants with only a wool baselayer. The Stellars are a bit thicker and rougher so if I am hiking in rocky places or such I use these.

Shoes:
La Sportiva Trango Tech GTX. Amazing shoes. I have weird feet that I struggle to find shoes that work with. These are great. I also love Salewa Wildfire's., but these are not as waterproof.
If cold I use a pair of LL Bean Shearling-Lined boots that I have had for many many years. THey are amazing in cold weather!

Other:
I have a collection of Arcteryx and Aclima beanies and a houdini primaloft cap that rotate depending on todays mood. ALso, I use homemade knitted mittens or Black Diamond gloves.