Welcome to the Biocube club
I have some thoughts for you based on my experience.
I’ll preface it as well with the fact I’m a gear junkie and have bought multiple iterations of upgrades myself.
Don’t rush to upgrade everything. The biocube is a great tank out of the box. There are some popular upgrades but they are mostly not necessities. I would a go slow and buy equipment to address a specific issue not just because you read or hear that something is “better” than what you have. Advice I have a hard time following myself

Among many other upgrades, I’ve got the intank media basket and I do feel that’s a worthy 1st upgrade as it directs the water flow onto the filter floss very efficiently and frees up half the middle chamber for whatever you end up doing (refugium or whatever).
As for a skimmer. I have the Coralife v2 skimmer. It works. It’s noisy. And probably not necessary especially not early on with a minimal bioload and CAREFUL feeding.
Reaearch quarantine tanks and that process if you haven’t already. It could save you a lot of heartache but most people don’t bother.
Don’t worry about your pH. It’s fine. a value of 7.8 to 8.3 is the “recommended” range. pH ties in more with coral growth so if it’s in range you’re doing fine.
Ammonia is going to be there at this stage. Even with live rock and sand until the bacterias metabolism balances to the bioload. My cycle took a about a week with 25lbs of live rock and 25 lbs live sand. I also dosed BioSpira to kickstart things as my live rock wasn’t closer to just cured rock and not all that live.
You can check out my build thread as well to see how my journey has gone and feel free to DM me with any questions. I’m new too but am a few months ahead of where you are so may have some useful info.