I did the race category in 2019 and will be in the adventure next year, not because it wasn't fun - it was a total blast - but made for some longer days trying to chase as many points as possible and having done it once, decided to take it a little easier this time around.
As for fuel/spares/tires - there is plenty of advice on traveling in Baja and the XL isn't really any different. I carried 10 gallons spare fuel and never needed it. I will carry 5 gallons next time because it is Baja and just because there is a gas station on the map, doesn't mean it has any fuel. The rally notes each day usually point out any fuel challenges for the route ahead.
Tires - I ran General Grabber all terrains - including spare - and had 2 punctures on the same day. Plugged one myself and then stopped at the next tire repair place to get them both properly fixed.
Spares - never needed anything but there are Autozones in Baja. I didn't carry any spares as such - just little things like extra fuses etc.
No forums that I know of but check out the Baja XL News and Support Facebook group if you haven't yet.
The spirit of the event means you can follow the route, or not. Off road a lot or mostly on road. Decide to stay an extra night somewhere to go fishing and catch up later etc etc. There is an offical camp at the end of each day and hanging out there was a lot of the fun.
Let me know if you have more questions.