I started out on a rigid MTB back in the late 80’s. It was a Bridgestone MB2. I later owned a Surly 1X fully rigid single speed. I really enjoyed riding both of them. The simplicity of the single speed was wonderful and I rode every where my friends did on their FS bikes. I had to work harder going uphill but my bike also weighed a lot less. I’m a fan of rigid MTBs and single speed MTBs. Everyone should own/experience one in their MTB journey. Here’s a couple more recommendations. Full disclosure, I own a Surly Krampus but I have the version with a front suspension fork. I have another hardtail so I’m seriously considering replacing the front suspension fork on the Krampus with a Surly rigid fork. Good luck with your search.

https://surlybikes.com/bikes/karate_monkey. 27.5

https://surlybikes.com/bikes/krampus. 29er

These two links default to a pic of hardtail bikes. Scroll down and you’ll see the choice for the fully rigid versions.