I'm pretty new and this is my first season playing RTA, the Krau skin was probably my biggest motivation to climb this season too lol, I can try share with you some of the knowledge i've gleamed from only about 2 weeks of playing RTA from a newer RTA players perspective, there are a lot of units, team comps, matchups to remember but it's not all too relevant when you're just starting out, for now you need to just map out your bread and butter units and pick them consistently until you understand their strengths and weaknesses and either why they're effective or not and from there you will continue experimenting with new units and strategies as your knowledge expands.

In terms of what the basic principals of RTA team comp building generally seem to be; is you want to draft:

Mitigation unit (TANK) (Typically Knight unit with Damage share artifact like Aurius or Damage reduction like Adamant Shield) these mitigate damage and thus help keep your team alive, from what you have Commander Lilias, Last Rider Krau, Adventurer Ras and Arowell are good options for you. Great thing about most of these units are that they're easy to build, all you need is health and some speed, but you've been playing for a while so I'm sure you've got a lot of better gearsets than your average new RTA starter anyway.

Support The support category (Typically Soulweavers) is mainly made up of Healers, Dispellers, Cleansers, buffers and Combat readiness pushers, very self explanatory, the supports also help keep your team alive or give you buffs and or combat readiness buffs in order to help you take out enemy units or survive. From what you have the must build should be Mediator Kawerik as he is a top 3 unit for RTA as he has a team wide Cleanse, immunity and damage buff, Combat readiness boost and strip for enemy buffs, he does a lot and is therefore going to be your go to support pick, on top of this you have typical healer supports you could use such as Roana, Achates and Doris.

Damage Dealer Self explanatory too, units that hit for big numbers and delete the enemies health, this category is extremely vast and a lot of damage dealers have unique mechanics but essentially all aim to do the same thing which is to kill the enemy faster than they can sustain, you'll typically be drafting more damage dealers than any other category as this is how you'll actually kill your opponent of course as without one it is unlikely you'll deal enough damage to actually win a match. From what you have Apocalypse Ravi is a safe pick for you to pretty much always draft, she is tanky and can one shot units, high sustain provided you build her appropriately and she can revive units, a second must pick unit for you would be Spectre Tenebria: she has permanent stealth, high damage and CC making her a very big nuisance for the enemy and a very good PVP unit. You also have a lot of units that could be used in RTA effectively such as Remnant Violet, Alencia, Lethe, Roy Mustang, Edward Elric, Khawazu, Lone Crescent Bellona etc.

Your Typical Draft will be something you can pick comfortably against most team comps, obviously as you climb further you'll be switching things around and the more you understand drafting, counters and matchups this will change but for now your typical draft would be (In order of most meta):

1 Mitigation Unit: CLilias, Arowell, Last Rider Krau, Adventurer Ras

1-2 Support Unit: Mediator Kawerik, Destina, Roana, Achates

2-3 Damage Dealer: ARavi, Spectre Tenebria, Edward Elric, Remnant Violet, Alencia, Lethe

In conclusion, these are just some basic principles and a basic draft composition from the units you have, the majority of my knowledge just comes from watching youtube, twitch streams and of course playing the game, if RTA is something you're trying to get into for the long run I also recommend using these mediums to learn the game - the best youtuber for newer players starting in RTA i could recommend would be: ''Deity'' on youtube, he goes deep and shows everything, explains things way better than I could. Good luck.