Returning after some time and feeling pretty lost and weak. Thoughts on which characters to focus on?

Bennett, xingqiu, fischl, kazuha

Then yaoyao.

Build demdro team: alhaitham/tighnari, fish, koko/yaoyao, flex.

For diluc, upgrade kazuha, zhongli (both lvl 90, stack em/hp, then you need reactor:  xongqiu, kaeya, rosaria.

Hope that helps.  Also try pulling charcters  furina, xilonen, yelan(for diluc)

Leveling up Skirk should be your first goal imo.

Alhaitham, Zhongli, Fischl, flex -or- Alhaitham, Zhongli, Kuki Shinobu, Xingqiu

Skirk, Mona, Chongyun / Ganyu, Xingqiu / Kokomi

Yoimiya, Kazuha, Bennet, Xingqiu

Strongly consider Escoffier on her rerun banner to add to your Skirk team.

Columbina + Any nod-krai 5 star will be a very strong core for a new team.

if you can get columbina before her banner ends, go for her and zibai and you'll have the most premium team out there with columbina, zibai, illuga and either gorou or zhongli. You're going to need more than one team though, so I would focus on Skirk next. She really wants escofier, but until her rerun you can run her with mona xingqiu and chongyun. Thats the strongest team you can build right now, the rest are quite dated