If we're going that far from the original request, I'll throw in https://smile.amazon.co.uk/Stray-Magic-Dystopian-Fantasy-Apocalypse-ebook/dp/B07N5ZVNLY
There is no romance between FMC and MMC, MMC adopts FMC, as a wizards familiar. There is no shifter content, but the FMC does have a hidden analogous status.
Much more common in sci fi then fantasy.
Stray Magic: A Dystopian Fantasy (Faerene Apocalypse Book 1) by Jenny Schwartz describes a world where supernatural creatures invade and take over. Sort of a problematic book.
There is a series that takes a rather unexpected turn when fairies slaughter most of humanity...
>!Darkfever: Fever Series Book 1!< Trigger warnings for...well, everything really. Don't read it if you can be triggered by ANYTHING.
I feel there are lots of books where vampires took over.
https://www.royalroad.com/fiction/26534/vainqueur-the-dragon I note. Also on the second https://www.amazon.com/Stray-Magic-Dystopian-Fantasy-Apocalypse-ebook/dp/B07N5ZVNLY/ref=cm_cr_arp_d_product_top?ie=UTF8&pldnSite=1 ( Becoming a familiar of a gryphon )
Not strictly litrpg, but does have levels and growing abilities.
https://www.royalroad.com/fiction/39152/alternate-eyes Reincarnated as pokemon, sometimes with a trainer.
https://smile.amazon.co.uk/Stray-Magic-Dystopian-Fantasy-Apocalypse-ebook/dp/B07N5ZVNLY Not very litrpg at all. Earth is 'rescued' from an apocalypse, and a 'lucky few' are taken as mages familiars.
https://www.royalroad.com/fiction/51151/im-not-the-hero Does a sidekick count?
https://smile.amazon.co.uk/Stray-Magic-Dystopian-Fantasy-Apocalypse-ebook/dp/B07N5ZVNLY There is no litrpg in this.
Following a 'monster apocalypse' - the MC becomes the familiar of a griffin.