I tried setting up this question as a Multiple choice but I think it would be unfair to have people pay to have their suggestions added. Instead I will award M17 or more to each recommendation if I actually try it out (buy/download/subscribe) and I consider it helpful towards my goal of learning at least basic Mandarin Chinese by EOY 2024.
I'm a native Spanish speaker and I English very good. I have tried learning other languages in the past like French, German, and even Chinese, though I've forgotten most of everything. I usually go with websites offering structured courses and apps like Duolingo. I mostly default to reading about grammar and pronunciation and because of time constraints I also like to download some audio courses and podcasts. I haven't tried language lessons in a group setting because I like to start at my own pace, but I could see this would also has its advantages. I'm not sure what has helped me the most overall in the past.
Long story short, I would appreciate your recommendations to make this attempt successful.
Sorry if it's overkill, but here are the useful links I've collected over the years from various extensive Mandarin learning advice guides, from the Duolingo forum before they shut it down
General course apps/website:
Duolingo (as you mentioned)
Unoffical Duolingo stories https://duostories.org/zh-en
Archived Duolingo lesson notes https://duome.eu/notes/en/zs
HelloChinese https://www.hellochinese.cc
Memrise https://www.memrise.com/en/learn-chinese/chinese-course
LingoDeer https://play.google.com/store/apps/details?id=com.lingodeer
SuperChinese https://play.google.com/store/apps/details?id=com.superchinese
ChineseSkill https://play.google.com/store/apps/details?id=com.chineseskill
HSK Online https://www.hskonline.com
TOFU Learn https://www.tofulearn.com
Dictionaries:
Practice/immersion
Discord Language Sloth
HelloTalk https://play.google.com/store/apps/details?id=com.hellotalk&hl=en
Language Reactor https://chromewebstore.google.com/detail/language-reactor/hoombieeljmmljlkjmnheibnpciblicm
World Stories https://worldstories.org.uk/lang/mandarin
Other resource lists and forums
Stack Exchange https://chinese.stackexchange.com/questions/1120/resources-for-learning-mandarin-chinese
Heavenly Path https://heavenlypath.notion.site/heavenlypath/Heavenly-Path-d9be1806465b4525afeb132d1079194c
Misc
If English was written like Chinese https://www.zompist.com/yingzi/yingzi.htm
Here are some self-study resources I used while learning Chinese:
-Pleco: This is probably the best Chinese dictionary app in existence. It has a decent built-in dictionary for free, more high-quality dictionaries for which you can pay, and a purchasable flash-card feature.
-Anki: This is a high quality spaced-repetition flashcard system.
-LingQ: This has a huge amount of free listening and reading practice available for many languages, along with a built-in dictionary system and a (last I checked not free) memorization aid system. I merely use the free components.
-The New Practical Chinese Reader line of textbooks. They were fine; I mostly chose it because it is available in both simplified and traditional characters (I prefer traditional).
-The free app "Chinese Grammar" on Android (https://play.google.com/store/apps/details?id=com.chinesegrammar) has some nice, concise explanations and examples of common Chinese grammar patterns. It can be a useful portable reference.
祝你成功！
One more which I forgot: hinative.com . On this site (or its companion app) you can ask questions about languages or post your attempts at writing or speaking in them and receive answers and corrections from native speakers. Most of the functionality is free; you can pay for tickets to make your questions show up more prominently, but answering other people's questions about your own native language also seems to bump up your priority enough that you really shouldn't need to do so.