Someone will no doubt be along soon enough with an advanced mathematical formula for creating the perfect deck…as for me, I use them all.

Complex math formula inbound:
Nah, just kidding. It's simple!

To me, the main guideline is to follow the creators' intention. There are cards that open a gate (burst and normal) and cards that don't. The first one has 3 categories: high frequency, mid frequency and low frequency. Seals can only be shattered on the first two. tl;dr : your mythos is quite balanced with the expansions you mentioned. Only the low frequency gates open too much (twice the amount they should), but that shouldn't be too much of a problem.

You can also try my method. The main problem to me with a 200+ deck is that it's hard to shuffle and keep in one place properly. Then there is the increased chance of freak mythos-streaks, which comes with such a large number of cards in place. My solution is to stick to a 'primal' state: 10 normal and 2 burst for high freq locations, 6+1 for mid and 2 for low. That's the same number you'd get by just using base game and DH + IH. The result is a well balanced, much more convenient mythos deck. In order to achieve this, you have to separate the big deck into locations, then randomly choose the preset number of normal and burst cards for each of them. Then finally, reduce the number of 'other' cards (2 doomers, surges) by the same percentage as you did for the other mythos - you should end up with about 7 doomers and 4 surges.

Also, since you're using Lurker, you may want to add all of the double gate openings and consider them 4×2 major openings on the main board (so you only have to choose 2 burst and 8 normal card for them). They are nasty and unique, thus should be part of the deck at all times imo.

Manishtusu…if that's easy , I don't want to know hard

For shuffling a big deck, a reccommend breaking your deck down to multiple smaller decks, shuffling them individually, then randomly recombining.

In my deck, there are 288 cards…

First, I lay out six sets of three decks (each with 16 cards)

Second, I roll 6d6 and place each rolled die next to its corresponding "set"

Third, I roll each placed die again and the result 1-2: first deck; 3-4 2nd deck; 5-6: 3rd deck determines which deck I shuffle and select a card.

Repeat two more times and you'll have 18 cards. Shuffle those cards and place them in front of your card holder/display and you're ready.

Seriously, it takes 5-10 minutes and you have a well-shuffled decks.

The Professor said:

And then first Mythos phase of the game you draw The Story Continues….

that made me laugh…

I have to tell you, having played nearly 100 games, I've NEVER drawn The Story Continues card.

The Professor said:

It's inevitably the first card I draw after shuffling. I think I have a talent.