Very simply, the level of stress depends on how often you work in the rooms. I worked in ambulatory endoscopy and rotated between phone calls, pre procedure and recovery (sounds like you have those skill down already). The phone call nurse was assigned to circulate and head to the rooms if an injection or tattooing was needed, or if her help was needed to break a laryngospasm, if a patient aspirated or something (rare but it happens). She sat at a desk all day making phone calls with a walkie talkie so she could communicate to the charge nurse if she was unavailable due to being on a call, and the charge nurse would cover. Nurses would not constantly be in the rooms - we had endoscopy technicians assist the GI with routine procedures.
Now, the rooms require a lot of technical skills in terms of taking forceps, nets, snares, injection needles, etc. in and out of the endoscope biopsy channel, putting the specimen in the jar and labeling it, etc., but it's not hard at all, just a new skill. Most doctors are patient as you're learning but it requires a bit of speed obviously because the colon may spasm or during an EGD, they don't really want to stay down in there too long. Your responsibilities would involve #1 obtaining specimens, polypectomies, you may tattoo, inject Epi, you may insert clips and use hot forceps and snares with electrocautery to control bleeding, etc. #2 patient positioning, #3 therapeutic maneuvers such as abdominal pressure and #4 help out if they need a jaw thrust, medications drawn or suction or something. It might get more complicated if the place you'll be working at does ERCP (usually not outside hospital but not sure how it works in the UK), EUS, fine needle aspiration, dilation or ablation, etc.
I do not know the eye surgery world at all, but endoscopy definitely is not stress free, I'll say that much. It's usually like a factory and very fast paced. Though it sounds like the weekday schedule would be better for you. I say you go for it - I find GI so fascinating and there's constantly a ton of research surrounding it.
Personally, I find it is what you make of it and there's plenty of room to grow too. I went from a staff nurse at an ambulatory endoscopy center to the manager of an office based endoscopy suite (where I also recovered patients), and now am a clinical coordinator for a small (1 procedure room for now, but we might be expanding to two) endoscopy suite where I manage a recovery room nurse and a scope technician, maintain accreditation, etc. and I'm the one in the room doing the procedures with the GI. We also give remicade IV infusions during cases for IBD patients. They get them 4 times a year but if we can save them a trip, why not?
Hope that helps a bit!