[Ben Jenssen]

Just some thoughts.
A bit random.
You say it's outdated and you want to move on.
What do you want to have that you don't have now?
Is something wrong with how it's working now?
(This is something you should be asking yourselves, and something we should know to be able to give good advice.)

I'm no expert, but I don't think you can get the 003 to work with a modern mac and mac os. I can understand that you like it with the faders and ins and outs and you're familiar with it, but if you want to go current with mac and PT, it's out.

The good news is that audio interfaces nowadays are SO much better sounding that they leave the 003 in the dust, and they're cheap.

So, either you stay with what you have if it's working, or you go all the way. That's how I see it. You get a mac that handles a current Mac OS and Pro Tools 2021, and buy a perpetual license or go subscription. I don't think there is an upgrade path from PT8LE.

Perhaps like this:
Get a used mac mini, like $500.
A small interface, maybe $250.
Cheap screen, a old TV, cost almost nothing.
If you have money left, there are lots of cheap controllers out there if you want to have faders.
You'll have a modern up to date mac, and subscribe and pay monthly for your usage of the latest Pro Tools.

Oh, and the latest PT can open sessions from 20 years ago here. But I have experienced some that don't. Haven't finished investigating. I have lots of stuff from way back PT5-6-7-8.