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I read a similar plot in a smut/romance manga, it is pretty wholesome and I recommend it. It's called Yandere Mahoutsukai wa Sekizou no Otome shika Aisenai - Majo wa Manadeshi no Atsui Kuchizuke de Tokeru if you're interested. Do not be deceived, the story and plot is pretty good if you give it a try. ML is also a reasonable yandere and there are a lot of character development. It's basically the same as this manhwa where they have a big age gap (FL (20) 12 years older than ML (8)) but FL clearly did not see him as a potential lover but rather treats him as his own child (even considered adopting him). What happened was, FL was fighting off a dragon but she wasn't strong enough to kill it so she used her time magic combined with making herself into stone and got swallowed by the dragon. She timed it 20 years as well bc it was said to be the remaining lifespan of the dragon. In the present, FL is 24 and ML is 32! The rest is up for you to read!
My first time commenting, so sorry for bad grammar.
Camping rn, this has a great potential