Missed one.  After reassembling and testing, this one worked but had a strange problem.  I had to press hard on the joystick right and it always jumped several locomotives each time.
Opened it back up finding I missed some dendrites shown below.  Cleaned it and it works OK.
On a different controller, I had cleaned these battery contacts.  It worked two days ago, but not today.  I successfully tinned the positive terminal, but the negative spring was too far gone.  Filing took off half the spring.
So, I covered it in solder braid, draped over the spring.  The solder braid is soldered to the PCB.

After going through all my Dynamis stuff, I thought I was seeing things.  I really have TWO dynamis control boxes??  They both work OK.  I also have two of the EZ controllers with the dial that can only talk to 9 DCC locomotives (ID numbered 1-9).

I have no clue where I picked up the second Dynamis system.  I was probably buying more hand controllers for $30 and they threw this in for free.

Shays were meant to 'in real life' operate tight switchback, poorly built, temporary logging railroads.
Some even rode on wooden rails.  A lot of those abandoned logging railroads are now Jeep/ATV trails.

In real life, they used link/pin couplers and obsolete railcar frames they stripped down to haul logs.

You will be using knuckle couplers which may uncouple in tight curves, depending on what you're coupled to.
This is the defect you will see in using your 18" radius.  You can also buy 24" radius later to fix this.

I've thought long and hard about this since I'm also getting old.
I also look at the tools I've picked up for cheap over the years off ignorant kids selling on Ebay.
Getting an axle quartering jig for $10 along with $300 in other specialized tools means only one thing:  Another Model Railroader died.  That would be the only way I'd part with these rare and expensive items.
Those old-time tankers I spent weeks assembling, painting, and custom decaling.. only pried from my cold dead hands.
There comes a time when money is meaningless.  If you don't have a younger friend or family member to pass these to, call your nearest Model Railroader club.  You can search the Internet for groups that will purchase your whole setup at once, but they are there to make a huge profit and are likely to wait until you are dead to put out a low-ball offer.
I am going through my collection and I'm 1/4 finished assigning a monetary value to each item.  Some are still unassembled kits I haven't gotten to.  Tichy Train Group and Accurail items are more valuable than blue-box Athearn.  Leave that list for your named executor so they don't get cheated.  Look on Ebay if you don't know what things are worth - you'll be amazed.
Every time I think I've had it, something brings new life.  Recently, I had open-heart surgery.  I only expected to last a couple months, but now I have years.  It would have been a mistake to unload my Model Trains when I thought I was a goner.  Nobody visits me, so I converted my Living Room into a gym.
I've got endless tasks ahead of me as there is no end to Model Trains.  The layout is never done, and there's always cars and buildings to put together.
Sad to see someone giving up, but it's your life and your choice.  I just saw an article about using spackling compound for brick lines.  I need to try that.

It's also important to observe ESD precautions.  If you are not set up to work on sensitive electronics, do the work on 50%+ humid days, wear cotton, and ground yourself to a metal cabinet or pipe that is also grounded.

The defects found are usually the controller going nuts like a three year old child running the joystick.
A different problem is the unit shutting off when bumped.  While you have these apart, clean off the battery contacts with alcohol.  If that doesn't work (like if a battery leaked out), use a small file to get some shiny metal.  Wipe on some machine oil to keep the filed contact from rusting.  Be sure and also check the removable 'Lid' contacts for any corrosion.

Repairs were all done on the back of the board.  It looks like they (China) didn't clean off the flux after they mounted the thru-hole parts.  Dendrites are fine crystalline hairs that form in humid conditions using residue underneath layers of old flux.  They lead to shorts or resistances that mess with microprocessors.  In short, it's poor workmanship that leads to infant mortality.
The two pictures below, one is a controller where one pair of batteries goes dead overnight - powered off.  Another is the IR receiver signal line that only works for 5 minutes after being turned on - then it disconnects.
After repeated cleaning cycles of scraping off the old flux, rinsing with solvent alcohol, then using compressed air were these cleaned into reliable operation.  Keep doing those steps until all the white sticky residue is gone.
The backs of these units were held on by 4 screws (longer ones to the battery case side.)

This is a success story on how I repaired 4 IR wireless handsets I gave up on 10 years ago.
They all work now.  I bought all these off Ebay over 10 years ago.  I suspect the reason I got them so cheap is because they were not working right in the first place.  So, these are the worst of the worst.