What I did recently in a similar situation with 1L starter was to let it ferment out completely, stick it in the fridge to flocc the yeast, decant off enough liquid to get it below about 12 fl oz, and then swirl the rest and pour into a beer bottle, cap it and store it in the fridge. It was refrigerated for a week this way, then I took it out before brewing, slightly popped the cap to vent pressure, and let it warm up to room temp on the counter. There was a little pressure, but not much. I pitched it into a batch that was around 1.060 OG, and fermentation took off within about 12-24 hours as expected.
In the future, if your starter is too big to pitch fully into your batch and you WANT to pitch all the yeast, just stick it in the fridge as I said above, and then once it settles you can decant a lot of the starter wort off and pitch the more concentrated yeast slurry.