Just noting they (millions and millions of cars on the road) have very low tension valve springs.

I know this is not the same, BUT, not too many years ago I ran dirt track cars in a class that was limited to flat tappet cams. Big solid cams and big springs, 7500rpm. Here was the cam break in procedure after refresh. I always put new lifters in, even on a used cam. We would start it up and warm it up, set the timing and shut it off and run the valves. Next stop was at the track for hot laps, qualifying and racing. Never used any procedure. Never wiped out a cam lobe or lifter. Ran thousands of laps on multiple engines, camshafts, spring pressures and everything else. All of this was done with VR-1 20W/50. My point is, if the valvetrain is set up correctly, you won't have any issues. I believe you are way overthinking it.

My previous crane cam had doubles springs. My new Comp Cam required only one spring .I'm glad I don't have to remove the inner and take it apart again.

I am surprised that nobody else has asked . . . . . what does your engine builder say to do ????? That would be the first choice of procedures especially if you expect them to stand behind the engine..

Was the engine dynoed ? If so then I would run it for 30 minutes then take the filters off and cut them open to make sure nothing stupid is happening. If the filters are ok put two now ones on and add a qt of the 10W40 to each engine. The go run it fairly easy for an hour or so. After that go with the VR or Mercruisers oil.

Just my opinion,

My engine builder said use the lucas 30w break in oil, run for 20 -25 minutes varying between 2000 and 3000 and you will be good to go for the cam and rings Then change oil and filters and use the high zinc valvoline vr1. He used to have a dyno...If there was a dyno locally I would have done it that way. A lot easier for break in and if it blows up on the dyno (it wouldn't me my fault) lol. But this time no dyno.
This guy does all kinds of engines even Farrari, Jaguar, He has done my engines in previous boats but he had the dyno back then.

Ready for 1st start-up I read this in another thread. I will follow these instructions on break in. The cost of oil is a lot cheaper than engine parts.

Prime the oiling with the distributor out, drop in the distrib. Fire it up, watch to make sure youre making good oil pressure, juggle timing to a "good enough" spot. hold it above 2000 rpm for at least 20 minutes. Watch oil pressure, watch coolant temp. If there are oil or coolant leaks, try to ignore them as long as pressures and temps are tolerable. Coolant temp can run pretty warm on this first run.
After 20 mins, vary revs some, but stay below 3000. If it's happy, then slow to 1000 and fo a quick tweak on idle speed stop screw and a quickie with the timing light and idle mix screws. Then shut her down.
As soon as it's under 115 degrees, drain the oil into a CLEAN pan. There should be VERY LITTLE sparkle (because your Jomar filter should have caught and kept most if it).

Refill w fresh oil (still break-in oil).
Do a second heat cycle and heat up by running at 2000 again. THEN you can slow it down, hook up your vacuum gauge, and start finding your best initial ignition advance (adjust for max vacuum), then adjust idle screw to desired idle rpm. Then adjust idle mix screws for best vacuum. Repeat several times until you like your baseline. You want to get it running on the idle circuits as opposed to running on the transition slots..
NOW you've broken in your cam and lifters.

Good luck!

Better for first start up on the ground before installing it in the boat ??

That’s the way I do mine.

Its nice I have a friend with a forklift but how many times can you borrow it lol