To avoid the hasels of the US lines at AMS stopover in Iceland both ways over night or for a few days and be done with the long lines .
At AMS you are a Schengen pasenger and can arrive 2 hours before the flight. You then go to KEF and arrive domestically. At KEF you clear EU exit control and have less of a hassle the reverse going to Europe clear EU Customs at KEF and go to the Blue Lagoon to get over jetlag spend a night or so then continue to Europe. Book an open jaw ticket.
No worries about connecting in KEF with crowds. Its worth the extra cost.