No it doesn't, the official build has all the kernel modules. On community builds like these from hnyman and KONG for instance it depends on what the volunteer in question has available in time and resources to provide and maintain all kernel modules for a specific build, or just the build itself.
Exactly, if you're in the experimenting phase to find out what you exactly need, an official build might be easier get started with experimenting. However community builds like the ones from @hnyman, @KONG or @ACwifidude for instance are more tailored to the specific device, but there are slight differences which they all explain clearly in their first post in their topic, and during the topic as well.
Yes there is: KONG pro. You have to decide for yourself what you trust. Most volunteers/contributors for community builds in open source software have a reputation. They built that over the years. The ones I mentioned are very active and seem very knowledgeable. I have no reason to distrust them, but is that the same as that I trust them? A general rule I mostly use; is the source provided with a description or some sort of explanation how I can use the source and make my own modifications as I see fit? Then I see no reason for distrust. You could turn that around and say: "that means you trust the builds" but that's for everyone to decide by themselves.
This looks like the journey of 95% of the alternative firmware users I guess.
OpenWRT does VLAN's out of the box. Link aggregation is something different. I consider link aggregation for OpenWRT a niche. It's possible, because Linux has support for it, but it's not a standard functionality. One might wonder how setting up a LAG would actually be of any benefit for a device like the R7800. You would need at least 2 ports of the integrated switch of the R7800 to set up a link aggregation to another device. This means you only have 2 other ports left, besides WiFi. WiFi will not ever saturate this aggregation. You haven't mentioned what you want to achieve with a link aggregation so I'm assuming you want to create one for speed with a NAS?
Time in the docs means gathering knowledge but it may get overwhelming sometimes. Bonding has been done before on OpenWRT, but I don't know it's current state.
You could create a topic in Installing and using OpenWRT with a clear description of what you want to achieve with VLAN's and link aggregation. People will likely respond and give you an idea what's a start. Link aggregation is usually used for 3 reason's:
- Increased throughput
- redundancy
- both
Now I'd like to add that I'm getting off topic here, this thread is about hnymans build, you asked questions, I gave some answers. I'd advise to create a topic in Installing and using OpenWRT with a clear description of what you want to achieve.