You mentioned not knowing what you need until you actually need it. Well, I'll just share my path to prove your not wrong, and sometimes ya just gotta pay and play until you figure it all out...

You can watch lots of youtube and other training video's but you never get to see every detail until you try it. I bet I'm not the only one that has learned this stuff the hard way... One thing this has taught me though is how automotive components and systems actually work and it has given me a good background into how to troubleshoot without an electronic device telling me what to do. This is probably considered a ridiculous amount of gear, and it doesn't even include all the Fluke and other meters i started with and stuff I've accumulated. The moral of my story is ---- you probably won't ever get out of this with 1 or 2 devices. Technology evolves! (My background is 35 yrs experience and retired Electrical Engineer) I wish you the best of luck in your journey, remain committed and persistent if you decide this is what you like!
Here's just the last 3 years of diagnostic stuff i've acquired and played with:
BlueDriver Pro OBDII scan tool (2018) $100
Autel MaxiPRO MP808K (2021) $865
PicoScope 2205A (2022) $270
Topdon Pheonix Plus (2023) $1012)
Topdon BT600 Battery Tester (2023) $152
Topdon Topscan (2023) Free for being a product tester ($60)
Topdon T-Ninja Pro (2023) $499
ZT-702S Oscilloscope Multimeter (2023) $70
Innova 5610 (2024) $350
As you might notice, 2023 was a busy year for me with Topdon equipment. COVID and working from home gave me a chance to "experiment" even more than normal
In the end, learning was more important than any specific diagnostic tool, but in hindsight there are some I would/could skip and others that could be combined to a single tool. Expense really drives things, especially when you're just starting out. Inexpensive is not a terrible thing when you want to learn and don't have the funding to take the short cut. But, the Snappy scanners are pretty nice
