So I think I can give a pretty unique perspective on melan hunting, or at least what I think can contribute to gaining a lot of Melans^^
long rant about melan hunting
Shiny/Melan Hunting Backstory time (useless info but may help)
I've hunted with both Arceus rank (for 2 months now) and without(almost 2 years). Ive used dozens of z-crystals, hunted with both a shiny charm and uber charm constantly activated and used hundreds of day care passes and egg supplier eggs on good bonus days (sei/niet) and I can honestly say that while Melans are fickle I've seen both good/ bad and unbelivable chains.
I started shiny hunting in October of 2016 and my first melan was hatched in November of the same year.
I have hatched 36 melans in my shiny hunting on PFQ (2 years and 1 month)
heres where it gets tricky
23 were early (an early melan for me is one that is before the long chain bonus kicks in)
7 of those early melans happened this year in November alone (skewing the data terribly) since they were with Arceus rank but Arceus rank is useless before the 1000 egg mark lol
I even got a melan Legendary on a 1/36 hatch
most of the rest fell within the 1-4k egg hatch range
Combee was 4125 eggs before I got my Melan Female (4 male Melans)
Gothita took me almost 4k eggs to get a complete shiny line (3)
And Hoothoot was almost 9k eggs for 1 Melan (that was with Arceus rank)
It may look like I'm just lucky? So many early melans, an average of 1 Melan every 3 weeks! only 1 really bad hunt?
What If I said that on average I hatch 5000 eggs a month?
(I'm #8 on the site stats for eggs hatched)
Or that most of my shiny hunts have been within the 500-1500 egg range
Bonuses are great, the more bonuses the better! What I think matters more in the Melan hatching chance however is hatching eggs and making those bonuses work for you. If you're not hatching the max amount of eggs per day then your not getting the most out of your bonuses stacking.
If you decide to get Arceus make sure you can make the most of it. Hatch your 139 eggs per day (or more) Use that z-crystal on Sei days, stock up on daycare passes/egg supplier orders to use. Make sure you are constantly hatching eggs and keeping up that momentum. If you get burned out by long hunts, do shorter hunts.
tldr; Arceus is okay, but not as big a deal as some make it out to be, You can easily hunt Melans without it, especially if it will be a big drain financially / mentaly(stress) but any bonus is still a bonus, and bonuses only get better when they are used together while hatching as many eggs as you can per day.