In Poland, as early as 2005, flash photography in churches during any events was officially prohibited, but you could always talk to the priest beforehand, and if you were very polite, he would usually agree. I even remember a priest once agreeing to set up four studio flashes in the church. Back then, it was a real treat. An additional requirement: if you wanted to take photos in the church, you had to have a photographer's ID (with your photo), which was issued by the church curia. To obtain this ID, you had to take a two-day course that taught the ethics and behavior of a photographer in a church. This, of course, also cost money, but not a fortune.
But getting back to the point, if you're a guest, focus on the ceremony. There will definitely be a hired, paid photographer there. Act in such a way that you don't get in his way.
For a wedding photographer, there's nothing worse than a guest struggling with his camera and blocking the best shot.
The wedding ceremony is the most important moment for the wedding photographer and the bride and groom, and the shots are unique. It's impossible to replicate this. Afterward, things calm down, and you'll have the perfect opportunity to capture stunning photos with your Portra 800.
Please don't take this the wrong way. I'm not saying you're an amateur who would bother professionals. You certainly have class and refinement. I just strongly advise against taking photos in a church (if a professional photographer is hired).
Good luck !