Hey, good looking scape and fairly easy to get back on track in my mind.
Im pretty sure what you are seeing is as
@Connswater stated , the transition from emmersed to immersed growth. Generally every leaf that was emersed will eventually die and new immersed growth will appear.
I yet to be sold on any benefit of dry starts apart from the interest and experience of growing aquatic plants above water. As soon as you flood, you start a process that creates a lot of die back and as such invites algae issues.
My general experience and that of many is that when you first fill a planted tank, large significant water changes are a must. 50% daily first week, then every other day for a week, then every 3 days for a week or two then you can probably settle into weekly changes. I believe this is even more critical when dry starting due to the extra die back.
Your tank during those first weeks should be shining and any dead, dieing or damaged leaved removed as soon as you see them.
I can't see you mention any livestock yet so don't be shy with the co2, you can be a bit heavy handed and then wind it back for stocking if needed, this will also save tweaking too much, which should be avoided. (The snails you mention won't care about high co2, I'd be sucking them out at water change anyway before they establish!)
Don't buy a new light, wait and see how you go in 8 weeks, probably isn't necessary. Of course, if you want a shiny new WRGB then fill ya boots 😜 generally 6 hours is plenty and makes life easier when you are establishing a tank, 8 is the end goal once settled and grown in a bit.
You will get a lot more die off from all the plants, get scrubbing, be consistent and it will turn around. 👍