Thanks for your question.  I need to have three additional pieces of information:

1).  What kinds of trees were removed?

2).  When were these trees removed?

3).  What types of trees are still in your yard and/or in the yards of near neighbors.

Looking forward to your response.  Thanks for consulting us.

Thanks for getting back to me.

In looking at the pictures in your initial question, I was struck by the number of these intruding plants and how randomly they were distributed.  Additionally your statement about them springing up quickly (presumably this spring 2024) is not consistent with most turf weeds.  That led to my asking you a few questions.  Your answers confirmed what I suspected was going on.  These are NOT weeds in the typical sense of the word.  Rather, I believe they are tree seedlings.  That is why I wanted to know about the trees on your property.  Trying to ID tree seedlings can be difficult and not always correct.  I ran the seedlings in your first picture through two different keys.  The same name came up in both:  green ash seedlings.  However I can’t rule out the possibility that they might be black ash seedlings.  When you mentioned that you have ash trees on your property, things made sense.  It would be interesting to know the type of ash tree in your yard?  See:

https://www.dnr.state.mn.us/trees/black-ash.html

https://www.dnr.state.mn.us/trees/green-ash.html

Ash trees typically release their seeds in September and October. It turns out that when ash trees are stressed – think of hot summers and drought conditions (e.g., 2022 and 2023) – they exhibit a “mast year”.  This means that unusually high numbers of seeds are produced.  This might explain why you have not encountered these seedlings in past years.  See the following for an answer I gave in April 2023 involving a similar situation:

https://ask2.extension.org/kb/faq.php?id=825121/1000

 I’m going to guess that seeds from your ash tree laid dormant in the ground until this spring (2024).  Suddenly you were blessed with ten bazillion seedlings.  As you mow your grass this spring, you will be clipping these seedlings off.  Some more may appear in the next few weeks.  By mowing them down repeatedly, by late June to early July you should see little evidence of them left.

After you have mowed a couple of times, go ahead with your over seeding.

Good luck.  Thanks for consulting with us.  Please get back to me if you feel that my conclusions may not be correct.

Glad to be of assistance.  I can confirm that the tree shown in your most recent picture is a green ash (Fraxinus pennsylvanica).  Thanks again for consulting us.