To combat the overflow I start many days in advance and do 2 step starters with cold crash and decant.
Are you using a yeast calculator to estimate volume, growth and cell numbers?
BrewUnited's Yeast Calculator
Make a starter, grow the yeast, decant new yeast and dump that new yeast into a fresh starter wort. The second starter would be larger than the first.
If you don't use a larger volume of starter wort for the second step, growth rate will be severely stunted. See that yeast calculator I linked to, above.
An alternative to using larger starter vessels, you could save out 80-90% of the slurry from the first step, to use in your upcoming brew, then make a new starter with the leftover 10-20%. That will give you more growth due to a smaller inoculation rate. It's like making a double batch.
If you want to save some time, letting the first round starter crash while propagating the 2nd round, use 10-20% of your 1st starter volume (uncrashed but well homogenized).
If the 2nd round starter contains 10% spent starter wort (= starter
beer) from the first round, due to omit crashing, make your new starter a little stronger, say 1.045 to come out at 1.037-1.040 for round #2.
If in a pinch yes you could add some extra DME to your spent starter. I would dissolve it and give it a quick boil to pasteurize, then chill, before adding. A stainless kitchen pot with a well fitting lid works wonders for that. Chills fast too in a tub or sink with cold water.
I find using 2 liter starter vessels working well for me, using propagation schedules as in the example above. I can't easily place a 3 or 5 liter flask in my fridge, I would have to remove a shelf to get the required headspace which is not very practical.