go out, as straight as possible to near edge, at cardinal points, NSEW then tilt that 45' and do those 8 lines you likely will pass a possible spawn point

i would recommend using another seed in the link that Zep Tepi msg'd you... and set all the world details to off except trader... then enter your seed.... its what i ended up doing as i play often in bright daylight IRL and i couldnt see anything often... so i traded for an advanced torch..kinda a spoiler...kinda not :-)

My tip is dont sail more than 1-2 islands away looking for him he's usually on a blackforest within that zoneage. Island is a subjective term here cause some islands can be gargantuan and not so.

Mostly just explore, but he's always in a black forest, and his icon shows up at a greater distance than your map clears. So you can usually find him by sailing close to any black forest you find. There are three possible locations on any map, but only the first one you find will stick.

Or...
You can look up your map seed here: https://valheim-map.world/

I would recommend exploring in concentric circles around the spawn. I think part or the generation algorithm makes it so he's always in a certain range if the spawn and visible from a coast. The easiest way is to navigate in wider circles around the spawn.

If you don't want your world spoiled, post your seed, we'll have a look and point you in the right direction at least.