How to configure for spindle motor - DC brushed servo w/encoder step/dir

Yeah, I wouldn't mind getting into Mesa boards or the like, but that will make much more sense for a later phase on a much more capable machine. I have a reasonable means but not rolling in it enough to buy a full-on machining center, so I'm kinda caught in the middle for now.
For this little toy, lpt (and the accompanying hardware it already came for it) is as worth the effort as it's gonna get.

I'll try fiddling with Base Thread a bit tomorrow, just to see what happens.

don't change the base thread,
but you have to set the "gear-ratio" in the servo-driver.
For max rpm you also may have to increase the DC-supply of the servo driver

you may look here to find some answers

LinuxCNC is telling you the issue:

STEPGEN: Channel 3: The requested maximum velocity of 5006 steps/sec is too high.
STEPGEN: The maximum possible frequency is 5004 steps/sec.

This limit is because your base thread is too slow as Mike_Eitel mentioned

That is, you have a 10 KHz base thread (100000 ns), this can only generate a ~ 5 KHz step rate
It addition to reducing the base thread period, you can enable the stepgens reset mode (your axis stepgens have this enabled)
which will allow you to generate steps up to the base thread rate. These 2 steps should allow ~30 KHz step rates, that is, 6 times
higher than your current limit

6x the current rpm gets me up to 720, so that's still a good chunk slower than before and still a few times slower than I need.
So I plan to tinker with those settings just for the practice, but it's probably time to punt on this motor for driving the spindle.
I'll just connect a relay to LinuxCNC so I can switch on/off the old motor via the UI and vary speed via the step pulley.

It wasn't for nuthin' I guess. This servo & driver is proven to work properly at least so I can use it to run an axis on a future machine. This was unknown a couple days ago.
I want to experiment with it further and see what kind of top speed it can achieve with this servo on a signal generated by other hardware I guess. Honestly I didn't expect to bump up against this particular limit so easy with a PC but I'm sure there are valid reasons it. Maybe it's an lpt thing?
Thanks to all for the help. I learned some things.

Can you lower the resolution of the servo step input?

if 5000 steps/sec = 120 RPM that means you have ~2500 steps/turn

At 2500 steps/turn, 3000 RPM would be 125 KHz, outside of the range of most parallel port systems.