Yes, but that's not what you said above. You said "My MO for cancellations and rerouting is to find the best option that suits me and ask the airline to book it." Basically despite criticising others for their advice on here, your own advice was misleading in suggesting you could just demand what you want regardless, and then claim for the cost back if they refused. As you acknowledge now, that is clearly not the entitlement provided by EC261, and is not advice I think anyone should be following in those simple terms.

Also despite your comment, I have seen many times advice being given to rebook oneself and claim for long rebooking waits like the 3 days you note - although I would also say at least for BA and most of the routes they serve, delays for BA offered rebooking options of that length are very rare. Typically a 1 day delay or maybe 2 are the most that tend to happen.

At one end of the extreme I think the OP's example of easyjet only offering their own flights and only a rebook 3 days in advance is a strong case for booking yourself and claiming back. What the airline has offered is not really reasonable, or "at the earliest convenience" if there are many other flights with space on the affected route in the intervening 3 days. However, at the other end of the extreme if the airline offers you a flight say 6 hours later, but you find one only 2 hours later that the airline won't offer, would it be a likely case for you to win if you booked the earlier and tried to claim it back? Certainly a lot more grey in that situation.