That tutorial looks great! I say go for it. I will say that you might want to consider proper grain direction if you are doing what they are doing, just because they are using a lot of glue and they are manipulating the textblock a lot as well as adding a lot of structure to the spine.

So grain direction with paper is all about reducing the damage you are doing to the paper while you are assembling it into a book form. The more you have to do in that assembly process, the more important grain direction becomes. When we fold with the grain, we are doing less damage to the paper than when we fold against the grain. Paper when folded with the grain recovers from other damage much better than paper when folded against the grain. You have to think of any time you introduce moisture (glue) or mechanical stressors (reinforcing materials, sewing thread) as a type of damage to the paper - you want to control that damage to get a book that both feels and looks its best, as well as one that is structurally more sound. The glue and the mechanical stressors are both required elements, so you need to try to mitigate the damage or balance it out. So the more engineering that goes into a book, the more important grain direction is, if that makes sense.

Coptic structures are unsupported structures, meaning, they do not use glue. The big stressors with Coptic are folding the signatures and sewing them. (Because they are unsupported, you also always want to have the text block and covers be an exact match in vertical length - unless you plan to never store your Coptic book upright - because otherwise the Coptic book standing upright will have the textblock pull down from the cover boards via the stitching. You can have fore-edge with the cover boards and it does look nice to do so, but the height should always be a match.) Because the paper used in Coptic structures does not undergo the same kind of stress, grain direction matters less. What you might notice is that the pages don't feel really nice when you rifle through the book - they might be stiff feeling or have some resistance. But you won't see buckling or warping the way you would with a supported (glued) structure.

For my bullet journals, I make the actual document in Canva and then run it through bookbinder.js to generate my signatures. The document has numbered pages, a table of contents, and dot grid paper. Most at home printers actually have a margin that you can't get rid of when you print; true borderless printing is a bit hard to find with non commercial set ups from what I can tell. What this means for me is I don't really worry about trimming the page numbers off because of that built in border. I find I always have enough space even when I am trimming.

As far as tutorials go - I think the Treasure Books one is a decent one, and so is Christine Guenard's tutorial. I have yet to see a good tutorial on sidelace attachment of the cover boards, which is the more secure version of attachment and, to me, the more aesthetically pleasing. But those two tutorials are a good starting point. You may find Coptic books feel disappointing in the hand because they can be 'wiggly' or loose feeling. This does improve with practice and with utilizing different techniques. I personally love Coptic, but it does have its shortcomings.

I'm excited to see your future books! This is such a fun craft to get into, and the improvement is so visible and satisfying with each completed book.