Hi! Sounds like a fun day. :-)
I recommend Au Petit Suisse, near the Luxembourg Gardens: https://aupetitsuisse.fr/fr
It is an old, classic brasserie with a locals/neighborhood feel. Not touristy. Good menu. I am particularly fond of a salad they do with mixed greens, walnuts, pears, blue cheese and country ham. It’s delicious! They have good daily specials and all the classic basics, as well. And it is not expensive.
That said, it’s in the 6th, not le Marais…
Alas, last time I was at L’As du Fallafel (about 6 months ago) the quality had deteriorated noticeably from its glory days. But maybe I hit them on a bad day. It continues to get raves on Trip Advisor.
Also, if I may recommend some YouTubers, you might check out recent restaurant reviews on Les Frenchies channel. They’re a personable middle-aged married couple and they do nice reviews by restaurant type/cuisine, area of town, price point, and more. Their content is very current and fresh.