Well, I did the Best of Scotland tour but well before Covid (2018), still, I'll say that a waterproof (not water resistant) jacket is a must. I did not have rain pants with me and when I saw my guide pulling hers out of her pack on our day over to Iona I knew I was in trouble, lol. I would also go with 1 pr of waterproof shoes and maybe a non-waterproof pair as well. My non-waterproof shoes got soaked that day but I was able to dry them by setting them under the heated towel racks in the hotel and pulling the insole out.
I went to Orkney and Shetland in 2023 and am headed back there in a couple of weeks. What I learned from that trip was also to have a waterproof ball cap to wear under the hood of my rain jacket to keep rain off my face/gasses. I am NOT a ball cap wearer, btw! I had a non-waterproof one that got soaked in about 5 minutes even with the rain hood pulled up over it. This time I'm ready, hahaha!
For travel in April I'd add a puffy vest, or if you are cold natured, a puffy jacket that you can layer under your waterproof outer layer. I'd also have lightweight glove liners - mine are Smartwool.
I'm a light packer and generally go with 4 short sleeve tee shirts, 2 dri-fit quarter zip long sleeve layers and 1 long sleeve tee. Again, if you are cold natured, I'd probably get some of the 32 degree Heat base layers when they come in to Costco next Fall. I also take jeans which is what I'm comfortable in. I sink wash mine as they are a cotton/poly blend.
I pack all my outerwear in a compression cube from Eagle Creek. It fits vertically in my day pack so I will have it with me every day because the weather you get in the AM may not be what you have at Noon or later afternoon, lol.
Have a wonderful time!