Hmm, let me try to condense some common stuff.

1. Open back headphones do not leak that much unless you're listening really loudly, you can confirm this with dongles/devices that have those estimations (still not that accurate) for decibels or volume hearing levels, see here for more info: https://www.reddit.com/r/headphones/comments/11a5ysg/qudelix_5k_and_moondrop_kato/j9r0kp2/

2. Open back headphones barely have noise reduction/isolation. Whenever people use them outside, they'll get tempted to raise the volume to drown out the external noises and this is not good for your hearing health. Try closed back headphones (with ANC or not) or wired IEMs instead or better yet, TWS earbuds (with ANC or not) when going outside, those will have better passive noise reduction.

3. Earpads and ear tips can make a big difference in sound. Not to mention the comfort/fit factors.

4. So amps/DACs/etc. are pretty much made redundant by well-reviewed dongles (with physical volume control) now, just look at the specs of those dongles these days, it's overkill. Only get the standalone amps/DACs/etc. if you want a volume knob, microphone/speakers/etc. ports, et cetera.

5. That's pretty much it. If you want to learn more about speakers/subwoofers/et cetera, go to /r/BudgetAudiophile and other speakers-focused communities. For the most part, that side of the audio hobby is even more inaccessible due to the room changes that you may have to do to chase that "best" sound.

Here's a bit more info on dongles (with physical volume control) and static/hissing/buzzing/et cetera: https://www.reddit.com/r/headphones/comments/117ya99/why_are_balanced_cables_so_damn_expensive/j9eib7g/ and https://www.reddit.com/r/HeadphoneAdvice/comments/100lh5a/dac_for_new_mb_asus_and_new_headphone/j2icwxm/ and https://www.reddit.com/r/HeadphoneAdvice/comments/zn88xs/help_on_iems_pc_usage/j0fns20/

Look up oratory1990 (check the subreddit, /r/oratory1990, don't forget the preset parametric EQ list (mainly for headphones but there's some IEMs there): https://www.reddit.com/r/oratory1990/wiki/index/list_of_presets) and crinacle's articles on what amps/DACs/etc. are and what they do.

See here for more info about Qudelix 5K, parametric EQ, squig.link comparison graphs, AutoEQ, et cetera: https://www.reddit.com/r/headphones/comments/109meb5/eq/j40cy1n/ and https://www.reddit.com/r/HeadphoneAdvice/comments/118lcq3/question_regarding_iems_for_gaming_reposting_a/j9i8l1n/

Extra Qudelix stuff, like Bluetooth capabilities, et cetera: https://www.reddit.com/r/headphones/comments/11832uu/qudelix5k_hidden_gem/j9hq4fd/

Search up Dekoni, Brainwavz, et cetera for aftermarket earpads, don't know which ones are good now as some people report recent quality control issues with Brainwavz/etc. and so check /r/headphones and Head-Fi for more updated info on earpads.

Try Spinfit W1, FiiO HS18, AZLA SednaEarfit XELASTEC, Dunu S&S, etc. ear tips for possibly better fit/seal, noise reduction/isolation, and comfort.

The dimensions/sizing/measurements/etc. for most ear tips these days will be on Amazon/AliExpress/HiFiGo/etc. Unfortunately you have to blind buy the ear tips in order to get a feel for the dimensions/measurement/etc. that you like for your ears. Remember, this will also change with different IEMs as they'll have different nozzle length, nozzle angle, and so on.

Cheap ear tips (basically FiiO HS18, Spinfit from some AliExpress storefronts, Chinese websites like MTMTaudio/et cetera, and so on) and possibly better soundstage: https://www.reddit.com/r/HeadphoneAdvice/comments/1157giz/upgrade_imaging_with_balanced_cable_iem_or/j90a003/

Here's a bit more info on ear tips (talks about narrow/wide bore ear tips, effects of sizes of ear tips, how to put ear tips on wide nozzles, et cetera): https://www.reddit.com/r/headphones/comments/10lly6d/how_to_make_sense_of_iem_tip_fit/j5xnyv2/

Also extra info on ear tips: https://www.reddit.com/r/HeadphoneAdvice/comments/10k210n/alternative_tips_for_aria/j5o2f8q/ and https://www.reddit.com/r/headphones/comments/10x7ast/suggestions_on_getting_a_good_seal_with_the_7hz/j7qrrcg/

This is how you should use Etymotics or really any IEM or TWS earbud to make sure you have that good/perfect fit for that vacuum seal (necessary for the bass to be properly produced): https://www.youtube.com/watch?v=8KwXEqe6Gq4

More info on headphones/IEMs and PC gaming: https://www.reddit.com/r/HeadphoneAdvice/comments/1151ezm/looking_for_1st_real_pair_of_headphones_mostly/j8zm0q8/ and https://www.reddit.com/r/headphones/comments/10zxpfb/why_are_you_using_openback_headphones_for_gaming/j85vv18/ and https://www.reddit.com/r/HeadphoneAdvice/comments/10ks4tr/looking_for_iem_for_gaming_and_music_listening/j5sjhyh/

Open back headphones noise reduction (scroll to the bottom half part of the comment), earwax clogging issue, parametric EQ, noise reduction for your PC/room, et cetera: https://www.reddit.com/r/HeadphoneAdvice/comments/115c99b/coming_from_iems_and_looking_for_headphones_150_eu/j92mpe9/

For comfort with headphones, if you have a big head, ear piercings, glasses, et cetera, basically anything that will interfere with the seal/fit of the earpads, here's more info about headphones and glasses and workarounds it: https://www.reddit.com/r/HeadphoneAdvice/comments/zkjjcu/are_there_any_headsetsphones_that_are/j00kjt2/

For TWS earbuds and Bluetooth usage, see here for more info: https://www.reddit.com/r/HeadphoneAdvice/comments/11565qg/need_help_buying_a_bluetooth_wireless_adapter_for/j9033u7/and https://www.reddit.com/r/HeadphoneAdvice/comments/10hdr2p/are_there_any_wireless_earbuds_witouth/j58keli/ and https://www.reddit.com/r/headphones/comments/10kai7u/should_i_keep_ear_buds_out_of_the_case_after/j5so87z/

Old school earbuds, earphones, et cetera are having a resurgence right now, more info here: https://www.reddit.com/r/headphones/comments/119tndv/please_explain_inear_to_me/j9o244r/

ASMR, supplements for focusing/brain fog/et cetera, delaying or cleaning earwax with IEMs and ear tips, et cetera: https://www.reddit.com/r/headphones/comments/118ny8q/identical_qc_issues_with_kz_iems/j9ifzz9/

Earwax, hearing health, et cetera with Etymotics and other bullet style IEMs: https://www.reddit.com/r/HeadphoneAdvice/comments/10iwqmt/does_insertion_depth_affect_volume/j5ifw7r/

With quality control of Chinese IEMs, AliExpress shipping times, see here for a bit more info: https://www.reddit.com/r/headphones/comments/10sna0p/i_have_it_engrained_in_my_mind_as_an_american/j7395yr/ and https://www.reddit.com/r/headphones/comments/zav570/is_hifigo_a_scam/iynrt65/