Would 8 bits be enough resolution ?
Do your drives have a step & direction input ? That's also an option.
The easiest & most reliable solution (may end up cheaper at the end of the day) is a Mesa 7i77, has 6 analogue +-10v outputs, 6 inputs for encoders, 16 GP outputs & 32 GP inputs. Tho you would have to pair it with a 7i92T. Might be a couple hundred USD but you'll get a proven, reliable and well sorted setup. Something to think of considering your 10HP servomotors (10Hp was right ?).
I designed a couple of add cards that work in conjunction with a cheapish aliexpress FPGA board.
I've run 7c81 mesa firmware connecting via an SPI interface with a RPi4, 7i90 firmware using an EPP interface, a modified 7i90 firmware that is more specific to the setup, tho this needs some mods to mesaflash and the linuxcnc drivers. These are designed to work with Mesa DB25 daughter cards. Will also interface to standard Parallel Port breakout boards.
It's also possible to use a RV901T with modified firmware to run as a Smart Serial card, my poor old card has copped a bit of damage whilst soldering & desoldering bits on and off. Or with the correct adapter it also works with mesa smart serial cards, tested with a 7i73 card.
The same hardware will run Ollies linuxcnc rio firmware, either direct SPI or using a UDP to SPI bridge. There's a basic config in the riocore branch for this board. I have a couple of mates that are going this route.
Quick link to what I've been working on.
forum.linuxcnc.org/show-your-stuff/51142...pga-card-9d60#289226
Could have gone physically smaller with the boards, but wanted to keep as much as possible thru hole as it's easier to assemble and can get parts locally.
As much as I'd like ethernet, that means a move away from 2 layer boards and a lot of other issues that would bring it close to the 7i92T price point.
At the time of working on this project the 7i90 wasn't available and even know postage Down Under almost doubles the price. Pre covid things weren't so bad so the mill uses a few mesa cards. This project is for the FrankenMyford.
I know this is really long winded.....
When repurposing the colorlight boards theres as few things to take into account, outputs aren't an issue as they buffer are already at 5v. The issue arises with inputs. After having looked at a few options I came to the conclusion none are easier, and none are going to get you away from desoldering & resoldering. Some will let you have free reign on what pins are inputs\outputs other options will have pins fixed as inputs or outputs.
Whilst they do seem to be a cheaper option, there are extra costs, even more so if you are the one developing the hardware, if you can find suitable hardware that someone else has had a go out it may be come out a bit cheaper.
This setup isn't too bad, there is a etherent module some one designed for it. The i5 bundle is the cheaper version. A plus for this is that i\o is pretty much free form, tho you still have ot consider the whole 3.3v to 5v translation.
www.aliexpress.com/item/1005002997753428.html
If I was in your position (working and not having an effed up back, this is a guess), I'd more than likely go down the Mesa route, even taking into account what the postage would cost Down Under. The big pluses, as I said before, it's proven, it's reliable, large knowledge base, and direct support from Mesa via PCW. Actually without his generous help I would never had been able to get as far with my project running Mesa firmware.
I'm not trying to discourage you from "rolling your own" but just trying to pass on some of the stuff I've picked up on the way.
