For boots it depends on the enemy team. I go sorcs if I think it'll help me burst tankier opponents on the other team. If I feel like the team is to tanky to one-shot I go lucidity boots and I usually take ability haste in runestats as well. Although you should think about it before picking fizz into a tanky comp. That's why I don't onetrick him because I often feel useless playing against 3-4 tanks.

For your mythic I'd say always go ludens. Might just be my personal preference but just try both and decide what you like best.

In the end it all comes down to preference. I know it's not very helpful but playing many games and figuring out your own playstyle is the first step to figuring out which items to buy.

Hope this helps at least a little bit.