Letting others read your work

I am curious... at what point do you include others to read your work? I tend to get really excited about my work and share snippets and stuff with some of my closest friends (who also have read all my work). But this time I want to tackle it a bit different.

I am working on a big project that has been several years in the making and finally comes to fruition. As soon as I started working on it, the story started flowing out of me... so you know it's a good one then.... and I kinda want to hold it close to my chest. I haven't even told my partner about it, it's all mine. However at some point, I want to include beta readers. Not my friends... serious beta readers that can provide me with honest feedback to create an even stronger story....

Except, I have no idea when to include them. I am still early on in the writing... first draft... reaching out to them now, could help me with making several choices that might do well with my audience, but might knock me of course.... reaching out to them after finishing the first draft gives them a better overview, but might result in me having to go back and make extensive changes....

NOTE: my story is based in ancient Greek with some mythology in it. I love the stories, but am in no way an expert in that field. But even though they are references, I want to make sure it makes sense to the reader. Would that be another reason to include them rather earlier than later?

I feel I am somewhat biased because of my enthusiasm, so I am curbing that now and am ready to hear your advice! Thank you in advance!!