My opinion, which has nothing to back it up except for inference and experience,
is that the airlines have set prices high for 2024 based on a) the Olympics in Paris
and b) the experience of (summer) 2023. What would be interesting to know is if
the demand/purchase data is confirming their analytics.
If so, you shouldn't expect to see prices come down. But if 2023 was a one-off,
and lots of people decide not to travel because of the Olympics, well, you might
start to see some moderation in prices. We have already seen some sales for
the saddle seasons.
As many other threads have noted, there are multiple ways to make your itinerary
work. Brussels is not very far from Amsterdam, nor is Dusseldorf; flying into those
cities followed by a short train ride might work. Booking an open jaw and then
getting a cheap intra-Europe one-way to BCN is another. Getting miles and booking
with miles is another. Being flexible with destinations and schedule is the most obvious.
It's important to know the refund/rebooking conditions of a ticket when you buy it in
this day and age.