It would be a good idea if the levels from the swap-able crusaders were counted as well when resetting, giving more idols and eliminating the feeling that I am upgrading my crusaders in vain.

is that really the reason Erika?
I think it's more then it would be really unfair to newish people who only have 20 people compares to someone who has say 38 right now. Lets make a little assumption that a crusader you don't even have recruited tight now costs a whole heck of a lot less to level 2,000 times then it costs to level someone that you are using even 25 times. So each and every reset someone like me made we would earn twice as many idols as anyone else.

You see this effect explicitly on a mission like Beauties and Beasts, with limited legal crusaders. It's technically legal to consider all illegal crusaders as 'benched' or even 'swapable' crusaders kind of, and none get leveled up when you level all at the end. Your idol count is half or less what one would expect, which of course is right.

I am using some of the swapped crusaders.
I do realise that it creates a difference between the newly started players and those who have been playing for ages. But is that an issue?

I think a cool solution would be to have it so all crusaders in the same slot were at the same level, so getting Artaxes to level 200 would also get Drizzle there, and only give Idols for the 20 visible crusaders like you do now.

I don't really think that's an issue, especially since testing out crusaders you don't normally use for your endgame formation requires you to be in the endgame already. What's the point of testing Groklok vs Sahsa before I can afford Drizzle, for example?