The design of the book design/tattoo is detailed in this thread.
https://forums.frontier.co.uk/threads/the-quest-to-find-raxxla.168253/post-10345698
According to the artist, Brookes only gave her the basics, it was actually she who developed the detailed themes.
The shield is a different commission made by Brooke’s to an established shield designer, their online work shows a repetition of similar themes eg Thors eye, half moons etc. but it’s evident this is a self portrait of Brooke’s, the axe depicted was another commission he made, a real axe which he showed off at meet-n-greets, and posted about on the forum.
The gates are an almost direct match to those depicted in the book/tattoo design. Their description by the artist mirrors that concept for the gates of heaven.
The whole St. Michael vs a dragon is likely a reflection of the Catholic story of the war in heaven, eg Satan as a serpent defeated by St. Michael. But there’s evidently cross over here utilising Norse / Celtic mythology.
In the artist’s (book cover) description she talks of the devil creating the universe. This ties in with the explaned theology but also Brookes “The Space Inbetween” centric plot.
It’s an interesting narrative reversal, and ties in with Milton’s Paradise Lost, where the fallen angels turn Hell into a perversion of heaven, and effectively Satans goal and achievement was to turn paradise into a perverse Eden.
Brookes did write about Eden very early on (in short fiction and interviews), where he cast Eden and angels as demonic and as wanting to destroy humanity.
In The Space Inbetween it was the Lovecraftian gods who created everything and our gods who stole it..
In arts there’s always a lot of cross-over and collaboration, so it’s likely Brooke’s took the artists themes and embellished and developed them into his own storyline, drawing upon inspiration.
Note Brookes makes little of the stories actual development, however it’s evident it meant a huge deal to him, considering the effort made to commit it to a back tattoo, but at one point in his blog he confirmed it was actually in a first draft state, but he was considering an alternate idea other publication…
During game development I followed Brookes on social media, where he did confirm he was brought up Catholic, but wasn’t a practicing Catholic. In his blogs he was very much into theology but not solely Christian and showed research into other divinities. In in one SM post he showed his collection of Cthulhu figurines, all in a row, and said ‘spot the odd one out’ which evidently was a traditional Catholic icon of St. Michael - so it’s presumable he may have had a deep affinity for the character.
In Elite Dangerous the character Hammer is based on Brookes, and has
Norse influences, where he’s described like ‘one of those old gods from the northern mountains’.