As a child in the 80s, had a lot of dental fillings, etc. Was dental xray radiation high back then? Should I worry about risk of brain tumor from that?

Don't worry. You were probably exposed to more radiation when on vacation at the seashore or in the mountains than in the Dentist's office. Columnated beams, fast films, lead shielding, limited exposure, all were designed to protect you. There was very limited risk, and the trade-off was an exponential expansion of knowledge. No, Dental X-Rays didn't,t then, and do not now, cause brain tumors.
Would not worry. Radiation exposure today is far less than then due to the sensitivity of the digital imaging sensor. However, back then the beam was columnated and lateral exposure was contained. Also, there were some fast films available requiring less exposure time. So, the risk should be low, and I would not worry. I was a child in the 50's(far worse then) and do not see this as a problem.
You shouldn't. If you've already been getting dental X-rays, there's nothing you can do to mitigate whatever risk you already have. However, there is no scientific prove that dental X-rays cause brain tumors. The dental X-rays are important in making decisions regarding certain diagnosis and procedures.
No, Not at all. Medical use of X-Rays is generally proven to be harmless. So you do not need to think too much about your past(mild) exposure to Radiation. Be proactive in cancer prevention though. This requires healthy eating, maintain healthy weight by eating healthful(plant- based) foods) and exercise, daily if possible.
No worries. Even in the 80's, not only was the dosage low, but the beam was directed in a very small circle only to expose your teeth, jaw, and gums. There are no concerns over the occasional exposure to diagnostic doses of dental radiation.