Not sure I understand your question. Both speed and usage would be an issue; I average ~70 mbps during the day but drop to ~40 overnight, not much to share before I'm close to our ISP's 6 (which was about half the price, no caps).
It’s very unlikely you have 70mbps sustained usage. Typically it’s an average of peak per X mins.
You likely could save money by sharing your Starlink with a neighbor and neither of you would have much noticeable impact to bandwidth. Your latency might get slightly worse because of possible “noisy neighbor” but gotta ask if that latency is worth the extra $.
No, I meant that's the average speed I get (so if we were splitting it I assume our speeds would each be half that when downloading / streaming simultaneously).
Yes technically, but I think you're unlikely to notice it in practice.
In the worst case, because of buffering, your Netflix video might take an extra 10 seconds to start and then you'd notice no interruptions (unless you're skipping back and forth). But even that worst case would be unlikely to really be an issue.
For example, are you ever interrupted by someone else in your household turning on another TV? It would be similar.
You wouldn't want to share with your neighbor if they are actively abusing this with torrents or some other constant high bandwidth activity, but ideally you'd pick a neighbor you already know well and can trust.
If its the latter, have you tried finding a neighbor or two and paying $60 each or $40 each?