Help choosing mesa daughterboards

Hello! This is my first time posting here. I tried reading as much as i could before writing and would appreciate any help!
I'm building a cnc machine and decided to go with Linuxcnc as the best option. But I am a bit lost on how to choose daughter boards and how to configure them. Right now I have mesa 7i76e, configured it and was able to rotate my stepper motor. Motors are closed loop steppers, but right now encoder is connected to driver board. (Motors are chinese NEMA 34 12Nm, with blue chinese driver HBS860H which is clone of Leadshine HBS86H if I understand correctly). As I understand, it is beneficial to close loop through Linuxcnc. As mesa 7i76e board has only 1 encoder and its used for spindle, I will need expansion for encoders.
Machine planned set up is as follows:
3 axis- 2 stepper motors for every axis (6 step/dir, 6 encoders?)
1 spindle- servo spindle with rigid tapping
and possibly linear encoders (3 encoders?)

Which daughter boards should i choose? I read that 7i85S is suitable, but i would need 2 of those. Which way is documentation about how to set up something like this? Or maybe there is some other solution?
Are linear encoders advised?\

I would suggest that encoder feedback from step/dir drives is not very
useful unless its from linear encoders. Also most closed loop step/dir
do not provide and encoder output so you would need to tap onto the
existing encoder/drive connection. This is not a good idea electrically
because the drives encoder input ground is almost always the motor
power ground (which is very noisy)

"1 spindle- servo spindle with rigid tapping
and possibly linear encoders (3 encoders?)"

A bit confused about this with the other statements. You don't need to have encoder feedback from every motor and linear as well. Currently the steppers use encoders ideally to maximize their output with the drives. Ie torque management.

Closed loop with linuxcnc typically means closing the position loop. Linux will use an encoder to make sure the axis is in the correct location. This is best achieved by measuring exact axis location after gear reduction, ball screw etc. An open loop linuxcnc trusts that the stepper moved where it was commanded at the acceleration and speed it was supposed to.

With all that you would need to make sure the machine has little backlash to close the loop with linear encoders. If not you may get a lot of oscillation as the motor hunts for the correct location. This can he hard to do with DIY

Thank you all very much for answering!
7i77 has analog outputs, which I currently have no use for, so probably will buy 7i85s. In future, if i upgrade to servo, will probably buy this board. To be honest, this project has already cost so much that a few mesas here and there wont change much.
Not connecting stepper encoders to linuxcnc makes it so much simpler. I will be able to get away with one 7i85s 1 channel for step/dir, 3 channels for linear encoders. Thank you very much guys!