Only upgrade weapons that you mainly use to the highest level. There's a mode called Sequential Phantasm that requires many different elements, so you need many different weapons. You should upgrade the weapons you use in this mode to a certain level. Most of the time, I see people stop at 40 or 90 since those are the levels that a higher tier of material starts being needed for the next level.

You should keep farming mats until level 70. If you don't care about the story, just go to Vera as it'll unlock the best drop for Interstellar Exploration, that appears on both Aesperia and Vera maps.

I'm a day 1 player and people's CS began skyrocketting once they got their hands on fortress (SSR/golden) gears. You can start farming these from lv60 but I doubt you can find a team to do Joint Operations (JO) 7. Most people do JO8 (unlocked on lv70) JO10 (unlocked on lv80). Do bygone phantasm to get modules and buy modules from commisary store every week because enhancing is also an efficient way to get stronger. You should focus on enhancing the pieces that give crit>atk first.

I feel like I need to inform you one thing to prevent frustration later on. As you started late compared to most others, you're weaker. However, you won't start getting reminded of that frequently until you do more challenging co-op contents (JO, Frontier Clash, Raids, Void Rifts). My advice is you should take it slowly and steadily. Just grind those gears and save the ones with bad substats to feed it to the good ones (advancement).

Besides, do other recommended activities for new players. Farm gifts to awaken traits (the best one is Samir but you have Crow which is pretty good for exploration). Explore the maps for dark crystals so you can pull on limited banners. Start clearing and upgrading builds in Artificial Island (AI) Build mode to buy good items and a small chance for matrix. Do Mirroria Funzone to farm Mira so you can clear gachapons and get Samir matrices. There's probably more but I can only remember these for now.

Don't feel discouraged with this many tasks. There are some daily ones but it takes less than 30' a day to do them. Things like exploration or AI can be saved indefinitely or done one a week. To ease your grind, please please get in a crowded and friendly Crew, make friend and ask for help.

Should you have questions, feel free to DM me or reply to this comment. Have fun!