As a general rule for a regular player, I’d say any lvl130 item could be considered an end game item.
However, it really depends on the individual player’s regular pearl supply. If you can get enough pearls each week/month to continually forge the 5 skull items, than you could start your “end game item forging” sooner than that.
Keep in mind that you also need pearls to forge your spells, troops and defenses.
My personal preference is to only really invest in items obtained after king lvl110. There’s a considerable difference between lvl100 king’s items to a lvl110, but not so much after that (specially pro items). The items I got when I was lvl110 are very close to the items I get now at lvl117, so that’s why I say king lvl110+ is good enough.
Considering my limited pearl weekly supply, I don’t have enough pearls for everything. So I prioritize my pearl spending like this:
spells - I forge everything until they have at least 20 forges on the usefull perks and at that point I stop focusing on that specific spell
troops - same as above
end game skull items (I only have a ring right now, so it’s not too expensive)
defensive structures - focus first on the most important perks (like range on LT and Heal towers), and just try to forge as much as possible, without overspending here, not to compromise the pearls needed for 1), 2) and 3).