First-time betta owner- any general advice/feedback?

Hi all, semi-recent lurker and first-time poster to this thread! I recently acquired a betta (my boy Siegfried) under sorta rushed circumstances and have had to fish-in cycle to get the tank established (and huge thanks to the betta care wiki on here for the comprehensive guide for this!). This is the first time I've owned a fish or set up any kind of aquarium so I'm trying to read literally anything to make sure I don't mess this up!

Happy to report that it is the third week on this programme and both the tank and Siegfried are looking well- ammonia, nitrites and nitrates are sitting at 0 consistently, though still doing 15% water changes 2-3 times/week, and Siegfried is happily active, curious and w good appetite! Second pic is him starting to flare because he saw me taking pictures haha

I recently realised that because I've being using pure RO water I will need to remineralise this for the long-term benefit of the tank and its inhabitants- I think this has been an issue for a nerite snail I added recently who's been a little inactive. I'm slightly worried about starting this now that I think the tank is nearing the end of cycling but figure it needs to be done- has anyone had experience with starting remineralising after setting up the tank?

I'd also love any feedback on how you think Siegfried's looking/ the tank set-up I have going! He's in a 29L tank w heater and filter and I've tried to plant it well and make it interesting for him but am I missing anything crucial? (It's not clear in the pictures but he has a cave and a lava rock with holes to hide in!) I also feel like I worry about Siegfried a lot because when I got him he seemed to have lost some colour in one of his pectoral fins- I think he's gaining it back but I'm constantly worried I'm missing something else with his health, so would love some reassurance! (there's a smudge on the tank over the top of his body on the third pic, it's not a scratch!)