Max wall O2?

Hi there, I'm new. I have a question that's been bugging me for a while and was wondering if you guys can help me out.

I looked after a patient with cryptogenic fibrosing organising pneumonitis on a night shift some time ago. He had not responded to the steroids at all, was rapidly deteriorating and had been told he had a max of 2 months to live basically. (He was declined for a lung transplant.) He was on 15L wall O2 via Hi-Flow nasal cannula with 35L and 49%, and his resting sats on this was usually at 85%. His sats dropped to 40% that night and I put out a code as although he was not for CPR he was still for active treatment, and the on-call doctor didn't know what to do. The resus team recommended putting him on 15L wall O2 via reservoir mask (which I put on when his sats dropped), AND a further 15L via the Hi-Flow that he was on previously. We used the oxygen from the next bed for this. His sats was seesaw-ing between 40-70% with BOTH on. (30L wall O2) The patient was more comfortable with both on so we kept them on until he passed away the next afternoon.

There was talk about him having too much oxygen, with CO2 buildup and decreased respiratory drive and all that. But he became palliative that night and I thought patient comfort was the main goal then.

What are your thoughts on this? I was trying to look for writeups on this but maybe I haven't been looking hard enough. What is the max amount of oxygen that we can safely administer for persistently hypoxic patients? And persistently hypoxic palliative patients? And through which non-invasive device?

Cheers. :)