What type of weather are you running in? In my area it gets just below freezing in the winter, and I run with a full length compression shirt, then a technical running tee, then a fleece jacket. Works in wind, rain and snow between around 12c and -4c. Leggings and head/face covering are also often needed.

Biggest thing for me is really socks and runners; socks are wicking insulated running socks with an ankle, shoes have GoreTex uppers to keep my feet warm and dry.

Something to also take into consideration is how hard you’re pushing/sweating.

Merino wool will definitely work as a mid layer, but you’ll likely overheat pretty quickly, and it’s hard to get the sweat out.