I saw Tony’s video and enquired in the comment section about the timing as based on Mirai knowledge you’d want to do the approach graft when sap flow is at the lowest so there is no risk that sap gets in between the two cambium layers that you want to fuse. I followed the same logic as yours and in my question to Tony I asked if regarding timing it was indeed a matter doing it just prior to fall vascular growth taking advantage of low sap movement during summer dormancy. Surprisingly, Tony replied to me saying that on the contrary he does it when sap flow is at its strongest. I don’t know if there is a nuance regarding species pro environment but Tony iz a professional with many years of experience under his belt and his opinions cannot be dismissed without deep thought.
We certainly need sap flow around to bring about the resources necessary for the graft to take.
Doing it at the cusp of start of spring growth fulfills this need.
I can only hypothesize why it works for Tony in summer in the north of the UK. Looking at the image below (taken from here), we see that in Bury, they have on average at this time of the year cold nights and cool to comfortable temperatures during the day. Even more, until the end of june they have very cold nights. Tony’s summer temperatures, particularly the night/day variation seems similar to what a normal spring would be in the PNW.
It definitely is not similar to my summers here in Montreal where we’re in the comfortable range at night and warm range all day.
I think this is something to consider. I imagine in Sweden you’re likely to have similar weather to that of the north of the UK so in your case it may work.