from my extremely limited experience (traveling in Y with a 13-year old, daytime IAD-LHR, overnight hotel, next day LHR-CDG; traveling in Y with a 17-year-old, BWI-EWR-BRU with arrival ~0800; traveling in J as a couple, SEA-LHR, 2130 departure and 1500 arrival; traveling in J as a couple, SEA-LAX-AMS-BCN, 1200 departure and 1900 arrival) I wouldn’t opt for an early morning arrival *at the destination* due to service timing, flight duration for sleep, and need to make non-routine arrangements (early check-in or an extra night) for first night accommodations … our hotel rooms were ready on arrival in both London and Barcelona; iirc I had reached out to the b&b host in Brussels and secured a room that was not occupied the previous night (thinking also that a midweek arrival, plus some ongoing renovations, made this easier)

if taking advantage of FI stopover on the outbound leg to Europe, you’re still looking at 0600-0700 arrival in KEF which only relocates the issue (that said, hotels there may well be more accustomed to travelers on this schedule)