I'm considering giving my PCs some Mythic tiers so that we can play around with the rules a little bit--something that's a bit out of the ordinary just to feel super shiny since they're dealing with literal gods.
I'm not sure how I would balance that though, currently I'm thinking +1 tier at the end of books 2, 3, 4, and 5, and one more at the end of 6 so they end up tier 4 for Unity, and tier 5 by the end of everything and postgame content starts.
That said, I'm not sure if/how much I should buff enemy groups to compensate for this, and encounter balance is definitely more an art than a science. Any thoughts?
I am planning to use mythic as well. I changed the campaign a bit and will introduce a second iron god like Hellion in the end of book 4. (I skip this alien rubbish and plan to use some other scenarios.)
So I have two legendary bosses infested with some godlike powers and the defeat of each of them will translate in a mythic level. (I have some sort of Highlander scene in my mind.)
They will end with mythic level 2 (not counting a 3rd lvl after defeating unity). I will make no changes in book 3 as one mythic level transfers in 1/2 regular level. Maybe, if things are getting too easy I will adjust some enemies on the fly.
My book 4 conversion will be a bit shorter than the original. So after the end of book 4 the group will be one level short of the suggested start of book 5 (lvl 12 instead of 13). But with 2 mythic levels I'am right in line with the final chapters.
That said I use the mythic levels for some new ideas and features but in a balanced way concerning the power level of the campaign.
By giving one mythic level after each book you will have to upgrade most of the fights in the second halve of the campaign. I wouldn't do this. If playing by the book, I would recommand using 2 mythic levels like me after book 2 and 4 and skip some encounters in these books to end with lvl 12 in the beginning of book 5. That's easy, practical, introduces the mythic feature and means mostly no changes at all.
Just my 2 cents