Hi welcome to the game! Before I post my thoughts, I highly recommend checking out the tonberry toupe website for their resources and guides. Trust me they are super useful and i really learnt how the game works and making the game more enjoyable :)
1) For beginners it is very grindy if you wanna catch up with the current content - typically needa put around 1 hour or more to grind the resources eg summon boards and story chapters so that you are able to earn tickets and gems and you pull for units that are able to handle the more difficult contents. When you have a regular roster that can clear less difficult content eg cosmos quests then you could slow down your grinding to around 30 mins a day.c
2) Honestly anytime is a good time to start. That being said, with Rude now available in the game I think it's a good timing. Many people pulled for him and he is a really good support unit for handling harder contents and have a decent longevity from what I heard.
3) Yes Cloud and Lightning are really good, but If you want lightning I highly recommend you aiming for the start dash banner coming in March next year with lightning full gear instead of simply just aiming for her BT in Reno's banner. Plus you've only got half a day left before the banner ends so unless you've got 175k gems ready now you really can't guarantee the weapon. BT without the other weapons namely cp15, cp35, EX and LD is actually unusable and can't fully benefit your team. To fully utilise the BT you will need all the weapons Max Limit Broken so that the units got the passives from the them hence when the BT turns start you could use different passives each time instead of spamming HP attack. I'd say pull for Rude, Cloud, Ramza, Paladin Cecil and the start dash. But really at this stage you just need 1 unit that is full built and can carry your team first, then you can start expanding your team roster by pulling on banners. Otherwise you can't really complete the quests and just get stuck. Anyway do look up the global timeline on the tonberry toupe website and check out some content creators on YouTube discussing what banner they are going for and decide what roles you are lacking and what you need. It varies with different people. Have fun!