I've been sporadically working on a CL tandem acquisition (1998 26" Santana Vision) and it's very close to coming together, but despite the geometry being pretty close to my road bike's, I feel like the cockpit is too low and stretched out. I was thinking of getting a shorter +30deg oversized stem off eBay (and also a 5mm spacer as my understanding is the Santana is taller than normal). Would such a high stem cause any handling issues, or even worse, ruin the tandem's looks?

Get what fits. Little else matters.

Angles on the stem mean very little, other than a sense of personal aesthetics.

The important thing is where the bars end up with respect to the steer tube, and the seat/pedals.

I presume the previous owner cut the steer tube shorter than your liking. I'd prefer an upward angled stem to using an extension and a more square stem. Fewer parts.

As others mentioned, get what works for you and your partner allowing you to comfortably/safely ride.

Is this a 1 1/8" threadless stem? Where is the spacer going? I do like to keep the stem only slighty above the steer tube as necessary for proper tightening.

Oh, and steeper angles will naturally shorten reach, so you may not also need a shorter stem.

Thanks for the responses. I have no experience with such highly angled stems so wasn't sure. I don't think the tandem's steerer was cut too much, there's still a large spacer underneath the current stem and I would put the additional spacer above a new stem. I just feel more comfortable with a more upright posture on it compared to on a half bike.

If the +30 stem is still pushing it, you can also get steerer extenders. They go on your steerer and place the stem higher. Might give you more options if you need them
https://www.modernbike.com/steerer-extenders

The steer tube extension along with a less extreme (~17*) angle would be my choice as well. Extended steer tubes are seen all the time on tandems. Aesthetics do matter.

Personally I wouldn't want to add a steering tube extension on my bike and looking at what you said above it doesn't look like you would need to do that. I'd keep the spacer below your stem and figure out what to need for extension and reach. This is a neat tool for figuring out reach and rise that I got from Velo Orange (I think) that should help you get what you need figured.https://yojimg.net/bike/web_tools/stem.php

https://yojimg.net/bike/web_tools/stem.php