Both these points are true about every belief system. So even if I assumed, for the sake of the argument, that what I'm saying constitutes question-begging --- why single out rationalists for being guilty of it?
My understanding - and I may be wrong - is that that is the whole point.
No perfect belief systems, only what fits the person.
If your lead was "my system has the same flaws as every other system....", but is there anything especially wrong with mine?
I would have agreed, yep, yours is like everyone else's, it works perfectly fine FOR THEM, glad I do not have to adopt it as my own.
Mine is much more troublesome, many little quirks, not so black and white, but I like it. You wouldn't, but you don't have to.
Me, I do not think I am the source of other people's happiness.
I know they are not the source of my happiness, so stands to reason I am not the source of theirs - by my belief system, you are free to feel otherwise.
Same with suffering.
Desire is the cause of all suffering.
You cannot eliminate desire in me, so not your fault if I suffer.
As for rationalists having a special problem with fallacies-
Religionists (I am not one) say "The Bible tells me so." - appeal to authority, straight up fallacy, but their whole system is built on that or similar.
Begging the question as well, but they do not claim to be rationalists, look to other places for Truth.
If your system is working, stick to it, just do not suggest it is flawless.
~ ~ ~
A special transmission outside the scriptures;
Depending not on words and letters;
Pointing directly to the human mind;
Seeing into one's nature, one becomes a Buddha.