Finally almost finished my craft after spending basically all my money, there's still an open prefix left is my only option to slam it?

Sell it, hopefully recoup the costs and then some and try again.

Imo before burning omens of light getting t1 desecrated mod i would have block picked some shitty mod.

In an ideal world, you would block pick a high ilvl mod you don't like so you could slam then whittle the other prefix as needed assuming the suffix ilvls are higher then omen of light loop the last mod.

Can you explain your thought process a bit? This has got to be one of the most cursed bow crafts I've ever seen. Why that base? Why desecrate for t1 inc phys? Why desecrate so early in the craft when it's not to block the afffix?
I honestly don't think there's much you can do to guarantee a decent result. I'd probably just slam it and pray, and if it doesn't get flat phys start over.

you have extra socket and 2/6 good mods, try a perfect and hope for flat phys, if not you can always annul down and start from scratch