Any question answered...

I too have UK13 feet. I gave up looking for clips large enough to work with my feet years ago. I have tried lots of different options, including DMR V8s with velcro straps. I have just stuck with clipless systems because I cannot get along with anything else.

For my touring I stuck with SPDs but keep a pair of Olukai slip-ons for wandering around the camp site.

I mostly use flat pedals. I did half of the TCR in 2019 with innov-8 trail runners. They were fine. I've also used five tens and other flat pedal shoes for long rides.

I use catalyst pedals, which are massive and designed for mid foot. If you pedal with the balls of your feet then normal flat pedals like DMRs would do.

I've never used clips and straps. Don't think they're necessary or useful, and I wouldn't want the hassle on a tandem. I'm riding a tandem this year and it makes it feel a lot less risky starting off knowing I can just put a foot down whenever.

Basically I think clipping in is overrated, unless you are racing or riding in a bunch. So much easier if you're touring to be able to walk easily.

Your feet are likely to bounce off the pedals a bit if you have too high a cadence off road, otherwise really no issues.

What do you want to know?

I mostly use flats off-road.
I have a few pairs of FiveTens but more recently have been using Shimano shoes (I can't remember the model, soz).

I don't really have issues. I do sometimes miss being able to drag my foot up (carrying pizzas back to the hotel one-handed is harder when you can't get the pedal into a nice starting position)

My missus used a cheap pair of FiveTens for Seven Serpents just last week. She had some knee issues but we couldn't get to the bottom of them. It could've just been simple overloading with not enough training in flats beforehand. Will use again but need to keep an eye on it.

Oh, I started out using Nukeproof pedals and those long midfoot ones (Catalyst) but I have massive feet and they got sore so I went for wider pedals rather than longer and have been using Crank Bros Stamp 1 and 7 in Large with no issues.

It's great only needing to worry about one pair of shoes. They do get pretty stinky though if you're keeping them wet and then hoping to use them for casual stuff later. I might look at different inner soles to help with this. More regular sock washing and less "spending all day and night in pissing rain" might also help.

EDIT: They're GR7s:

https://www.biketart.com/products/shimano-gr7-shoes

and it's actually the cheap PX merino socks that smell worse than the shoes after all the waterlogging.

Oh, I don't use clips or straps. Half the point of using flats is so you can bail off the bike easily when riding anything beyond your pay grade (which for me is basically everything)

I don't think I'd use trainers or hiking boots or anything like that because they usually have uneven outer soles that would sit in awkward places on spikey pedals. I like the soles to be flat so you can position feet where they need to be and not where the shoe wants to sit (because of some groove or what not)