Rental apts in le marais

There is no area in Paris that I would not qualify as being "reasonably safe", certainly nowhere in the Marais. However, rental apartments can be troublesome as many, probably most, are offered illegally. Reservations can be cancelled at any time, a good number are cancelled by the city when investigators remove short term rentals from the market place and fine the owners. At a minimum, look for the government required 13 digit registration included in any online apartment offering. It does not guarantee legality but not having one strongly suggests an illegal apartment.

I rarely offer hotel suggestions without budget guidance, but consider Citadine or Adagio apart-hotels or any of these for group accommodations (located throughout Paris):

https://www.edgarsuites.com/fr/residence/
https://hoteldulevant.com/ 5eme
https://www.lockeliving.com/en/paris/le-jardin-de-verre-by-locke
https://www.pepperandpaper.com/ 5eme
https://yooma-eiffel.com/en/
https://www.roidesicile-rivoli.fr/

Take a look at Apartments du Louvre. They have two locations—one is in the Marais.

I've used Apartments du Louvre & Cobblestone Paris over the years.

We stayed in one of these apartments last December. https://www.lebeaumarais.com/en
Highly recommend. Great location!

The Marais may be one of the most tourist dense places in Paris -- a huge percent of apartment shave turned to vacation rentals, old bakeries and butcher shops are now trendy boutiques -- you can get fancy ice cream at many locations. It is beautiful but very touristy and pricey. And it is certainly safe -- but then all of Paris is pretty safe as far as physical safety goes.

Pickpockets are ubiquitous wherever people crowd together -- particularly the metro and places like the Louvre, Orsay and Eiffel tower.

I've stayed at Apartments du Louvre and Citadines and liked them both. Highly recommend.

We stayed for a week in Vincennes, and I can't recommend that suburb more highly. It's on the eastern terminus of the Metro line 1, and it's a very short ride into the heart of the city. Plenty of good restaurants in Vincennes also. There are some lovely apartments in the Woods of Vincennes area, a very quiet and tranquil area. Pricing is quite reasonable by Paris standards.