You can have "New tank Syndrome." However, not always and dependent on load (yeah one betta doesn't produce a lot of waste).
I also am using https://www.aquavitro.com/seed.php - Seed - help quickly establish a bio load and complete the nitrogen cycle quickly. I've had good luck with similar products in the past.
I use the Prime to get metals and chlorine out of the water and then use Seed dosing everyday for 7 days and then every few water changes.
The filter has a sponge and bio-ball type plastic media as well. So that should help.
That Royal Gramma is nuts looking! I've seen them. Beautiful fish!
Yes, many different bettas. They can be expensive, well, relatively, but they live a long time and I really enjoy them. I've kept several in the past and Katie even had one before I met her.
Lincoln was $45 versus $5-7 for a normal betta. It's all the eye of the beholder. The store I got him from had a lot of bettas but the staff was super happy I got Lincoln because they really liked him a lot. They know my background and that i'd give him a good home.