@TheG I’m going to make about 6 points.
First thing
Nothing you have shown indicates your browser was synced with any other devices. If you had devices on the sync chain, you would see them listed. Such as:
Secondly
You’ll notice that you tried to exclude it, but in the spot that shows what’s synced, it shows nothing. 0 Bookmarks, 0 preferences, etc. You can see it in the bottom right of the screenshot you shared below. So this tells us that nothing you have was synced with anything.
Third
Be careful on making assumptions if you don’t understand what you’re looking at. Often these logs and areas are meant for Support and Developers to help when there are issues.
Fourth
All of that said, @Alice2095 should have been able to point these things out to you and helped you feel comfortable in terms of you thinking it was syncing to another device. If they didn’t understand this, then they should have at least come back to this topic by now. It’s sad how they mentioned would get back to you but then 17+ days passed with no response, even after you’ve replied back to them.
Fifth
If you ever have concerns that your data is compromised, you can email privacy@brave.com and they can try to help. If it’s related to Sync, you would provide them the username listed under brave://sync-internals and they could try to delete the information or possibly investigate.
You also always have the option to submit a support ticket, which you can do at https://support.brave.com/hc/en-us/requests/new
Sixth
In terms of crashes, if it’s still happening, you would need to get crash logs for it. Instructions on how to get crash logs for iOS can be found at iOS crashes: How to share logs.
On top of that, you need to provide more details. You mentioned crashing but then focused on sync instead. So if it is crashing, you need to advise what’s happening when it’s crashing. Also it’s important to know which version of Brave you’re using, which OS (so if iOS, which version of iOS?), type of device (iPad, iPhone, or what?), etc. I do know you shared it’s iPhone 10, so you got that. But nothing was shared on other details.
You can’t get assistance if you don’t provide details. As much as people want to help and make attempts, we shouldn’t have to ask a lot of questions.
Side Note
You posted this in the wrong category as well. You put this under Contributing which would mean you’re a developer or something of the sort and you’re attempting to make a contribution to Brave. However, since you were seeking help, you should have put this under Browser Support