The summon system has Basic Banner and Selection Banner paths. Selection Banner unlocks later and is better when you already know which Mythic unit you want.
Current extracted logic shows a normal pity threshold of 250 and a Secret pity threshold of 20000. Treat Secret chasing as a long-term goal.
Before spending Gems, decide whether you need a stronger team now or a specific late-game target. A pity calculator should be one of the first tools added after the wiki pages.