First, let me just say that I am not an expert in Belgian Blonde. I've never brewed one.

But, I do have experience with that yeast, and I prefer using it under pitched, at least according to Mr. Malty. I haven't used it since I started using calculators based on Kai's numbers. I used to like to pitch between 50-75% of what Mr malty recommended. I never got the character I was looking for when using the full rate. Also, all reports from Belgium indicate that the brewers there under pitch the Belgian style strains for abbey style beers. I think the one exception is Westy because they pitch the leftovers from Westmalle and it's reported that it's a gross over pitch for their batch sizes, but the culture may be in pretty poor shape, so who knows.

Next up is the consideration that higher OG will by nature produce more character. Your blonde is not that high OG, so it's not going to display Tripel levels of character, pretty much no matter what you do, so I think you are free to push the character pretty hard. Also, for competition, being overboard on the style is usually better than trying to take the subtle approach. Especially with Belgians. Do something to stand out sort of thing.

Lastly, sucrose. Use it. Don't invert it, don't darken it, don't use some expensive candi sugar or some other nonsense. Use pure sucrose. Beet sugar, cane sugar, either one. But I am of the semi-unsubstantiated opinion that sucrose is necessary to really get WLP500 to do the fruity thing that I like about it. When I've made all malt beers with it the beers just lacked a certain something in the yeast profile that my sucrose beers didn't. This can be difficult to balance the malt flavor and dryness of the beer without getting the ABV too high. This is the secret to Belgian Blonde IMO, the malty/sweet/dry balance under 7.5% ABV.

Hope that helps.

My normal process is to pitch @ the target for the OG before sugar, then add the sugar after high krausen.

I've always added the sugar in the boil, but I don't suppose it matters much.

1.073 is probably big enough for a tripel or golden strong to be honest. Especially if it dries out enough. You could definitely up the sugar to 10% and do it as a tripel as long as the color is light enough.

1.073 is my tripel,dries out to like 1.005 and is supremely enjoyable. Not boozy. But I do run closer to 20%sugar.

Only fermentation stall I ever had was a 20% sucrose beer with everything in the boil.

What is mio/mL in regards to cell count?