Based on the answers, the strongest fit is the Mamiya RB67. It’s a well-regarded 6x7 medium-format SLR with a waist-level finder option, interchangeable lenses, and a strong reputation as a serious system camera. Since 6x7 is close to square, it still suits someone who prefers 6x6 but wants flexibility to crop. One answer specifically recommends the RB67 and suggests looking for a good Pro-SD film back, as back condition can matter.

If you’re open to 6x4.5, Mamiya 645 is noted as one of the cheapest ways into medium format, with lenses that are relatively easy to find used; waist-level finders are available.

Bronica systems are also worth a look, especially if budget matters. Community feedback notes that Bronica bodies and lenses are often cheaper than competing systems, while still offering interchangeable lenses and waist-level options depending on the model.

Hasselblad is the iconic 6x6 choice, but it’s generally less budget-friendly than Mamiya or Bronica.

For night work, remember that many older TTL meters have limited low-light ranges, so very long-exposure photography may still require an external meter or reciprocity testing, regardless of system.