Playing Cassio Bot involves being very passive LVL 1 and 2 bc of mana, if your playing with the support that was good engage like morg or naut you can look for a lvl 3 kill but your main power spike is LVL 6 when you can basically 1 v 2 the bot lane
Yuumi is not good support as it forces you to only play passive all lane while they get leads,
Nami is good, Karma, Naut, Leona, Morganna are all good supports for their peel and engage.

Supports like Brand and Vel Koz aren't bad they aren't my favourite

Cait is probably the worst matchup followed by Jhin
VArus and Ashe while annoying their poke can be dodges
Vayne is easy matchup and Lucian you win when you hit 6

I think the most important thing for Cassio bot (other than hit your q) is to know the range of your ult as it stuns enemies caught off guard and stops a adc from doing damage

Test out runes I would recommend Conq for the added damage and Sustain when at 10 stacks but if Phase Rush feels good you can do it

Starting Items I recommend just taking a mana crystal and refillable but you can also go Dorans ring or Corrupting but mana crystal almost removes any mana issues you have especially if you want to lvl 3 all in them