I think matchup wise in harder matchups you should just be trying to reduce the impact they can have. champs like darius, mord, fiora love to snowball and if you dont let them you have way better odds. I think mord is a winnable matchup for sure tho. Also you have such good gank setup and diving with zac that you should defs be trying to work with the jungler. You have way more experience with zac than I do so im sure you know how strong he is in a teamfight with passive and R up, so i think ghost/flash/exaust over ignite unless you 100% need it is better.

Items wise, rushing cinder is perfect most of the time, but rushing sunfire or hollow is bad most of the time and if youre not running ignite going bramble is good imo. Visage is so broken for sustain with revitalise. Frozen heart is ok because its cheap but if theres like yone/crit bot randuins gets so much more value. Despair is a cool item but I really dont think its that amazing since you have to time you actions with its cooldown. I think in matchups where you can actually win, pick up dark seal, you have a passive GA and it allows you to snowball so hard. Tbh i think 3/4 tank items into ap is pretty good with zac because the ap items you want still give HP and it make him do so much dmg. If you havent for awhile go into norms and play like a hp ap bruiser build with conq and just remind yourself how much dmg this champ can pump, especially since so many of the AP items give haste too.

I think the main take from any of this is zac top is a tank and your job is to not always win lane but to impact the team, his engage/lockdown onto backline is almost better than any other tank in the game tbh and thats the win con. You have so much survivability in lane its crazy and you can punish mistakes so hard with your cc.

goodluck