I totally agree about the key being when the window opens--up to the hour if possible. There are websites that keeps track of all the award tickets that release on a schedule (here's one:
https://upgradedpoints.com/travel/airlines/when-do-airlines-release-award-seats/ ). United is one example of an airline that used to release awards fairly reliably about a year ahead, but now seems to release them when they want and mostly a week to a few days before a flight nowadays for the saver tickets on their own metal. But they are still okay for partner availability. I would also start by using award hacker to find the airlines and deals for the route you are most interested in. That gives you an idea of what you should be looking for:
https://www.awardhacker.com In my experience first class to Europe is difficult. I was lucky to get a bunch during the pandemic on BA (with hefty fees I was willing to pay), but they've pretty much disappeared now. It might be worth it for you to pay for an alert service or two to have them look for your ideal awards--understanding that it might never open. Expert Flyer worked for me for a while, but then they stopped doing BA flights for a while without telling me, which really tee'd me off. (I was paying subscription solely for BA flights at the time with all kinds of alerts). They fixed the BA award issue but I haven't gone back. I really like seat spy for its simplicity now. It was free for a while, but now I think they've started charging for upper class flights. My experience is mostly with the UK since I travel there 4-5 times a year. But you can usually use them as a base for other european flights if you are willing to pay the fees. I've had very minimal luck with Flying Blue awards at decent prices. They can be crazy. I think one way was 225k points in business for a flight I was looking at recently. Nuts. You could also think about hiring someone to do it for you. I know
@Calfan hired a company for first class seats to Japan (trip cancelled to COVID), but she was really happy with the service.