Dejnov forgot to mention that Cyclops has a passive that boosts all thunder himes's hp by 5%, which works even if she's just in sub, so she actually stays useful for a long time.
If you haven't already done so, also grind out the water epic quest - for whatever reason they decided to put Jormungandr as the first water epic quest which gives a thunder assault SSR weapon, not a water one.If you're still in SR weapon land, then farming the SR Axes from Ultimate raids will get you a working weapon grid fast.
This is kinda long-term and won't be relevant for a while, but be warned that thunder does not have any f2p SSR exceed weapons for at least the next year or so, so you will eventually hit a wall in power unless you get lucky with gacha drops. Even SR exceed weapons are hard to come by - if you are starting now, you have already missed an event that gives an SR exceed weapon, and there isn't another guaranteed one until next October-ish from Tartarus epic quest if I'm not mistaken. So, keep an eye out for any thunder exceed weapons you get and make sure you hold on to them (Perkele's in particular, 'cos you don't need to brick her weapon to get the exceed.)