Around ten years, four months, and a day later, ish
"Ferb? Why is there a big red button in our living room?" Isabella asked, hesitating with her hand over the button embedded in the wall. On one hand, curiosity beckoned, on the other hand, the last time she pressed a big red button, it was while she was visiting Candace at work and everything exploded into paint. It had been incredibly awkward to watch Vanessa's dad start crying.
Ferb came out of his workroom, smelling distinctly of motor oil and confidence. He took one look at the situation and made the decision to scoop Isabella into his arms.
"Huh?!" Isabella screamed, blushing as Ferb carried her to the centre of the room. He set her down there gently, before going to the wall and pressing the button.
Then, the lights went out.
"Huh?!" Isabella screamed again, before the floor started to glow in rainbow hues.
That was when the cannons appeared.
They shot fireworks and flowers into the air and Isabella did not know how they weren't destroying the ceiling, but she was too awed by the display to bother asking.
She didn't know how she didn't notice Ferb, but she didn't, not until he held her hand and offered her a rose. Isabella grinned as she took it.
Then he squeezed her hand as the fireworks started up again.
This time, instead of bursts of light, they spelled out words.
Isabella gasped at the question, turning to Ferb to find that he was holding out a ring to her.
"Yes! Yes, of course!" she screamed happily, throwing herself at him.
He stumbled backwards a bit, before catching her with a kiss.
The two laughed as they separated, before Isabella spoke.
"How did you even do this?" she asked. "Where is our ceiling?"
Ferb shrugged, saying, "a magician never reveals his secrets."
Isabella rolled her eyes.
"Should've figured. Though, I would love to see a magician who does rainbow rose firework cannon shows."
She froze.
"Oh no."
"Isabella?" Ferb asked.
"HOLD ON I NEED TO CALL CANDACE!" she screamed as she ran out of the room.
Ferb's confusion lasted for about two seconds, because Isabella ran back in to kiss him on the cheek and say, "I still say yes it's just that I have to prevent a time paradox."
Ferb didn't know how to react to that, though he was happy she said yes.
(Phineas only found out at their wedding, after years of denial.)