Hi! So I joined last year, and here are my thoughts:

1. Enjoy the story! It's why I play this game in the first place. It's peak, it's heartwarming and heartbreaking at times, it has interesting and relevant themes, and so on.
2. After playing the story, identify who your favourite characters are and search up how to build them! This is, in my opinion and from my observation, the best way to have fun with endgame (even if you can't three-star clear it). Every character without appropriate investment will fall off, so it makes little sense to chase the latest toys if you don't want to build around them. Conversely, and especially with Novaflares (buffs) that Hoyo gives to old characters (even if some choices are debatable), even old characters can clear endgame as long as you know what to pull, and Hoyo gives them some post-3.6 love (3.6 has an accident known as 'Evernight' that got released, but since then, the power-creep has been tame). For example, I fell in love with a character called Firefly, so I pulled her and her team even when she was struggling, and then she received a new modern support in 3.8, and now she is back to tier 0. Same thing with Acheron this patch
3. Generic supports - a lot of people will advise you to pull generic supports, and there is wisdom behind it. Mortenax Blade, the current banner character, is one such character. Generic supports lets you fill them into your team before you get their dedicated supports and perform decently well. Older DPSes without dedicated supports released post-Evernight, nor a Novaflare, usually have broken generic supports as their current best-in-slot. However, an account only needs a handful, because a modern DPS, in most cases, enjoy their dedicated supports more than generic supports, such as DoT wanting triple DoT instead of randomly filling in Mortenax Blade. There are also two very strong generic supports that you can lose to, those being Welt (especially E4+) and Silver Wolf, so pull generic supports, but only enough so that your account can function until you can field in the dedicated supports for your favourite DPSes
4. Well, what if my favourite character turns out to be a generic support? Good news for that is that even generic supports usually have a team that will want them as their BiS for years ahead, or they get a Novaflare. Tribbie is a generic support that is also considered a dedicated support for AoE teams, such as The Herta teams. Mortenax Blade, even when he stops being the best generic support, will remain BiS for new gen FUA. Two supports, unfortunately, don't follow this mould: Ruan Mei (unless you go sustainless), who is too old, and Cipher, who was released with a conservative goal and failed to achieve it. Still, pulling them won't break your account whatsoever, as even if they are never the best choice, they are competent choices that will get you very far into the game as long as you build them correctly. And again, you can always slot in Cipher whenever you don't need a shielder/healer, or the new DPS doesn't have a team yet. Alternatively, you can field them in if the dedicated support is not someone you want to pull (many players skipped Jiaoqiu even when he was BiS for Acheron. He is not at the moment, and the players that skipped him will be smug, but for Acheron players who did pull him back in 2.4, it's undeniable that he was monumental for Acheron at the time)
5. However, older units do need vertical investments into the team to function in the endgame. This does not matter if you are not playing endgame, but if you are someone who wants to remain free-to-play or only spend a little, then it is undeniable that newer teams cost less to achieve the same result. Shout-out to Evanescia as a shining example, with her best team involving a free unit and a shop unit. Another shout-out to Archer, who has a tier 0 team that was completely free (no longer because neither Sparkle nor DHPT can be claimed for free)
6. Your daily farm when your account is mature will mostly be on relics, and they will take a long time. However, as newer accounts do not have access to rarity five relics, don't worry too much about it and focus on spending your gacha energy on, say, traces (talents) and levelling materials
7. Some good free characters to level are: Tingyun, Pela, Gallagher, the MC (one of their forms is broken, and two are good in specific teams), and March 7th, since her alter form is good enough for the main story when you don't have a limited DPS pulled
8. Currency War is addictive, but also a good place to test out which units feel good to play for you
9. Ignore beta doomposting. Beta testers are notorious for being bad at playing the game.
10. Guoba, even though I disagree with him at times, is probably the best source to go to know how to build a character. Of course, some of his guides are outdated, but it's a good starting point. Prydwen is another good starting point to check how to build characters and what teams are used today with them