"Marriage is definitely in the cards," says the source, adding that even though the actress, 49, initially ended her romance with the Cali Group CEO, 43, because he wanted to take things to the next level, she's since had a change of heart.
"Jen was not into the idea of getting married again," explains the source, noting that she was previously wed to Scott Foley and Ben Affleck, with whom she shares kids Violet, 15, Seraphina, 12, and Samuel, 9.
"But this past year has been rough on everybody," continues the source, "and it's taught Jen, for one, not to take anything for granted."
According to the source, Garner and Miller are hoping to walk down the aisle around her 50th birthday next April.
"She likes the idea of a small and intimate affair with their closest friends and family there to help them celebrate," dishes the source. And although it'll be low-key — and likely kept a secret from the public — it'll still be very special.
Adds the source, "There will be good food, music, champagne and Jen's favorite red wine."
The Alias alum's loved ones couldn't be happier. "John's a great guy and he can really relate to her. They're both divorced and co-parent with their exes," shares the source. (Miller has two kids with ex-wife Caroline Campbell.)
"Deep down, Jen always knew John was husband material, and in hindsight, that was one of the attractions," the source adds, noting that tying the knot would also be the perfect "revenge" as Affleck runs back to his ex Jennifer Lopez.
Plus, "Jen's time apart from John has made her reevaluate things. She thinks she's finally ready to make this huge step and is very happy about it."