You will be the best person to decide that. Depending on what you want to do, back bending can be different. Maybe try these and then decide.
Nails - can you comfortably sit across a table from someone holding their hand into a position where you could apply nail polish? You would sit like this for 30mins for mani to 2hrs for acrylics per person.
Sitting on a chair behind someone laying on a couch (you can adjust heights of beds though), where you can reach and see their face. This would be for around an hour for facials or lashes.
When i trained i had bad knee problems. My teacher wasn't supportive but it didnt hold me back. I just adjusted the few treatments that caused me issues and I've now been doing beauty for 12 years x
Standing beside a bed at hip height. Maybe try an ironing board. 30-60 mins for massage.