No, it's very small, something like a 2mm hex. I need to take another look, but so far I couldn't get those small hex keys to hold onto it -- it seemed round. (Maybe it was stripped while assembling at the factory... I'll check the other pedal).

(The black plastic cap on top was indeed a 8mm hex).

Oh, I understand what you meant now: there's a hex nut around that small axle I was looking at. I'll have to find a very thin socket wrench as there is very little space around the nut. Thanks!

On my gravel bike, I have ZERAY ZP-108S pedals. These are 283 g and they've been just OK. One of the screws fell out and one pedal is a little harder to clip in and out of. The bearings have also needed some adjustments. To be fair, they've taken a few hard rock strikes. I also got these from Amazon but there are a few sellers on AliExpress as well.
https://www.amazon.com/Mountain-Clipless-Compatible-Included-Multi-Great/dp/B083V1K59R
https://www.aliexpress.us/item/2251832669914249.html

I'm using the same Zeray ZP-108S pedals and am pretty happy with them. However, one of the pedals came with too much preload, and therefore doesn't spin as freely as the other.

I've opened its plastic cap (see attachment) but can't figure out how to disassemble it further: the screw head inside doesn't seem to take a hex wrench -- the inside of its head looks round (see other attachment). Could you help?

Thanks all for the suggestions.

I use a 80mm Uno alloy stem, so the ControlTech Falcon Mini stem-mount combined with third-party bars sounds promising. I'll have to see if the stem dimensions are compatible. Not sure though, since the ControlTech website says they're only compatible with their own stems? But since it attaches around the stem tube it might work.

I have found this on AliExpress: https://www.aliexpress.com/item/1005006263236960.html These would probably fit my requirements, at least the length and spacing is adjustable. I'm not sure how much you can adjust the pad position.
But they are designed for flat aero handlebars, and I'm using oval-shaped gravel bars from Ziino: https://www.aliexpress.com/item/1005006624272139.html. I asked if they'd be compatible, and the seller told me he's not sure, so I probably won't try my luck.

Any other recommendations for metal frames with wider tyre clearances than the Tsunami Seaboard GR02? 42mm is a bit small. Ideally I'd like 50mm, also to be able to easily use fenders.

Yes, Novatec D411/412 definitely work; so far that's one option I've been offered, as well as an OEM Taiwanese model with 36T ratchet through Peter at xmcarbonspeed.com.

I'm just wondering about what sort of tradeoffs I'd be making. Esp. since opinions of Novatec seem to vary online, and I couldn't find any tests of the D411/412 even though it's been in the market for several years.

You can get a (much) lower price point than that from Peter, c.34mm wide rims, various depths, no holes in rim bed, pillar wing 20 spokes (better value & performance than sapim cx ray, apparently) with no-name "taiwan" hubs. Just ask him by email. I bought 4 such wheelsets from him.

Yes, I've emailed him already, and also got in touch with speerdercycling.com. I'm trying to understand the options a bit better before I order.

By the way, Peter appears to have only wavy rims in the wider widths I'm looking for. What's a good resource on the pros/cons of wavy rims?

Well, value and weight are also important
I don't know much about hubs yet, but it seems DT Swiss are a bit overpriced compared to other brands like Novatec ?