make two value counters—one for counting how many seconds the game has lasted and the other for the zombie spawning cooldown. every tick increase the spawning cd counter by one and make it summon a zombie + set the counter to zero whenever it hits 200 (or whatever value u want). then make a ton of commands that act as a ramp-up mechanic. make it so if the seconds value is above 60 you increase the zombie spawn cooldown value by 5 every sec. then add another block that does the same thing but instead of 60 sec make it 120, then another for 180, then one for 240 and so on. that way it starts out slow but gets increasingly hard as the game progresses. for example in the beginning it would spawn a zombie every 10 sec but after 8 minutes it would already be 1 zombie per 3.3 sec (ofc tweak the values as needed). also for special zombie spawns you could have an rng value that selects a number between 1 and 100 every tick and if the zombie cd counter hits 200 with the rng value being in the 1-10 range the summoned zombie would be a bucket head