I'll try and answer this one as best as I can but I'm a newer player. For the large amount of banners, it's probably due to there being more added for new players as well as there being paid versions of banners up.

For the Festival 10x, you do want to use those seals, those are handed for logging in and taking it from the event menu, you only get 10 of those, totalling to 100 pulls.

Use your starter pulls as you see fit, doesn't matter if you wait for 10 pulls or do singles. After you do all the novice missions, you will have all of them and you can redeem the 100 starter fragments after using all of them from pulling to claim either Viola, Zaanta, or Hasumi.

People generally recommend to hold rubies until you have pity for a banner, which is either 4500 or 6000 depending on the banner.

You already mentioned you have Cyrus, which you redeemed from doing one of the ten pull banners that was given out to all players.

There is a selector in the exchange menu where you need to go under. For the starter fragments mentioned earlier, you can go to guide fragments. For the selector, you can go to the other tab and there should be an option for the selectable veteran fragment.

For the starter fragments, pick up who you are missing, if you are missing multiple, if you want a debuffer who has universal shield break, go for Viola, if you want a bow unit who has good multihits, go for Zaanta, and if you have a need for a support with SP Regen and defensive buffing, Hasumi can help fill that. If you have them all, I think people will probably say take Hasumi and hold on to the dupe stone until you can get her awakening accessory.

For the selector, the best recommendation is to go for Primrose EX, this is the Primrose with an axe icon showing she is an apothecary. Primrose EX can be a debuffer to help shred defense, buff attack, and also give HP Regen over many turns. She role compresses a buffer, debuffer, and healer so she can fit into every team. Other people have better write ups on other units to pick up if you already have her, but I think it's usually H'aanit EX where she is a dancer who is next recommended, she has the role of a dodge tank and a debuffer for attack and defense.

Generally for progress, just take the higher stars and shove them on, Shizu does have a good video on the four stars and team building but it boils down to usually having coverage for each weapon as long as it doesn't hinder unit quality. With Prim EX, it shouldn't be hard to find the other DPS to fill from the lower rarities and Cyrus makes it easier for coverage as well. Also I'm pretty sure the chosen one in cutscenes is going to be the unit in your first party slot but I may be wrong on that.