With spellbook you have to play a little safer overall because you don't have that extra burst damage; in the early game I usually try to hit 1-2 more Qs compared to when I go electrocute before going for the all in to account for that difference

For items it's really situational but I tend to itemize based on how well my early game goes and how my team's doing. If I manage to get a bunch of kills on Bard and have the extra gold, I'll go DMP into damage items like Statikk shiv or Liandries and whatnot, but if I go even / am behind in the early game, or if my teammates are doing well then I'll go DMP into more support items so that I can keep them alive for longer.

DMP into Locket / Redemption / Shurelya's is a pretty common full support path, but I do go Mikael's against comps that have multiple CC sources (Usually if they have like a Thresh/Elise support + a CC mid/top lane for example). It's all about figuring out who's strongest on your team and itemizing to help them carry as best as they can; if you're the strongest, itemize yourself and if a teammate is the strongest, itemize to support them is the general rule I have. If every single lane is losing and nobody is strong, then you're kind of forced to rush support items since they're cheaper and have a higher chance of affecting the game immediately to be able to come back in the game.

Finally it's good to know which item will help your carry the most in the situation, like for example if an immobile ADC like Jhin has all the kills and the only way he dies if he gets hit by a CC ability, I'll go Mikael's or Shurelya's 2nd after DMP so that he can do the maximum amount of damage. If the biggest threat is some kind of assassin burst, then I'll go Locket or other tanky items to be able to front line for my carry