The letter you are receiving is typical.  When alimony ends, there is a large change in the child support amount.  Child support is based on income though, and not retirement and investments.  Child support is also based on employment and income that was "historically relied upon" to support the family.  This means if you have additional employment income now, the court does not have to consider that.  You are allowed to obtain additional employment or make additional money to try to meet your child support obligations.  I will tell you, you'll have to make that very clear to the court though because some judges fail to remember that part of the guidelines.

Discovery matters are complicated and require things to be done on a certain time line.  I can give you some basic help, but you might want to consider hiring an attorney.  Both parents are required by the guidelines to provide proof of their current income one another.  To reiterate, that's both parents.  Make sure you request the same of your ex.  This request is not a filed request, but can count against you.  If you do not provide, the other party "might" ask for attorney fees to request it through the court.  The other party is fishing for income because their motion will be to modify child support - trust me on that.

What you might consider doing at this point is sending a copy of your last couple pay stubs and request the same of your ex.  Income, not taxes, are what is required by the guidelines.  From that point, the other party will file a request for production.  This will give you 30 days to provide certain things.  They may also subpoena your employer for your income.

You can object to the request, but I would advise that you just provide the income proof.  If this were a filed request for production or subpoena of business records to a 3rd party, you could object and let the court decide the admissibility.  If you want to object, all that is needed by Kansas law is to send written notice of objection to the other attorney.  I recommend doing so via certified mail return receipt.  They will then file a motion to have the court decide

I'll try to help.

Yes you need to have reasons for the objections.  A hearing will be held to decide whether your objections are founded.  The law states that if you object, you need not provide the information until further order of the court.  Check KSA 60-245 for subpoenas.  I can't remember which one covers the discovery process.  You can object and say the request is an undue burden and you can also request payment for the copying.  You do not need to provide the records until that payment is satisfied.

Just keep in mind that child support does not depend on assets.  So, you can probably object to documents proving assets on that basis.  There's no reason for the other party to have them.

What I have done is objected just like you are proposing.  The judge then ordered that the extensive amount of data (like my employment application) be provided.  What I did after the judge allowed all the information, was went home and drafted subpoenas exactly like the other party and even asked for a few more things.  I got everything I wanted because I had a court order telling me it was okay.

Discovery is a broad topic and there are many things that can be discovered if they are relevant to the case.  The other party needs to prove they have some relevance.  You need to prove they are not relevant.  When you provide written objections to the attorney, just go to the court house and file that letter with the court to let them know you have objected.  Provide the attorney with a file-stamped copy.  I've been pro se for a very long time.  Attention to detail keeps attorneys from catching you in a technicality because many do not like dealing with pro se litigants.