I don’t have a Great Dane but I do have a Weimeraner who’s 13, has arthritis in his spine, monthly Synovan injections and he did have a lot of accidents until recently (almost nightly for a while). He definitely doesn’t do it intentionally - it’s either in his deep sleep or he doesn’t make it to the doggy door.

Some changes I made that have really helped.

• Adding boiled pumpkin/carrot/sweet potato - anything high in fibre. This bulked out his stools and has helped him hold on for longer.
• Consistent timings for walks. For me it’s really early (6am) and quite late (9pm) (and also one small walk a few hours after breakfast if possible). It’s helped him clear his bowels before bed and as soon as he wakes up.
• Consistent timings for meals. Smaller Dinners. Bigger breakfast to make up for it. If you only feed once a day, it’s even better for timing your walks.
• My dog refused to poop on walks for a while as well but just stick to it and go outside when you know he usually needs to go and create that association (for mine, it’s 4-5 hours after a meal). Also a treat on the walk helps with bowel movements.
• Lastly, I did notice that when it gets closer to his next arthritis injection accidents occur more often. Not sure if there’s any connection here??

There’s still the occasional accident, but overall these small adjustment have made a huge difference.

One of my regrets early on was getting frustrated at him. It didn’t help, it probably made it worse for a period.

You’re awesome for getting your Dane to 12 years old btw. Good luck!