My now 17-year-old dog (lab mix, also a rescue) started having similar seizures about 10 years ago, also about twice a year, but shorter in duration, more like 2-3 minutes. My vet did blood tests to make sure nothing more serious was wrong, and then advised against medication unless things got worse. He also said that if I was around when the seizure happens to try and get the dog to focus on me and to pet him. That always seems to make the seizures stop more quickly; the vet said that if I wasn’t able to get the dog to focus on me, that’d be a sign they were getting worse.
10 years later, my dog still gets the occasional seizure but is otherwise perfectly happy and healthy (well, for an old guy.) I should say that I’m not against medication or intervention generally — my dog has also had cancer surgery and is currently on anti-inflammatory meds for his arthritis.
Definitely let anyone who watches your dog know that this happens. Because it’s very scary to see it without warning even if it turns out to not be that serious.