Hey Seb! Sorry, this post was pending approval (no idea why), which is why it wasn't showing up here yet. Great to hear you're enjoying the journey, you've been sharing some amazing shots!
1. The time of year / time of the day does. Dave, who's pretty active here, has a chart that shows which ISO you should use, depending on where you are and the time of year / time of the day. To be honest, I've been keeping my light meter at ISO 2 and that will always get you very close.
2. I've personally never used a stop bath / washing between dev and fix and it hasn't negatively impacted my photos, so I just don't bother :D
3. I know, right? I do a quick was, <1min and my oldest HDPP captures still look like I shot them yesterday. Better for the water bill too.
4. In many parts of the world, there's (local) recycling stations where you can drop them off. Just ask your local council for guidance.
5. I personally use an oldskool analog light meter and sometimes an app called LIGHT METER (black and white logo with f/ 16 in it, it's made by a company called WBPhoto. I find it good in some conditions, but it's not very accurate in other conditions, hard to fully rely on. I always tell people that it's all about experimenting until you find something that works for you.
Happy shooting!
Martin