If you aren't experiencing any vibration or other tire problems, I guess I wouldn't worry about the brand. I don't know about the price you paid. I had never heard of the Sumotoma brand, but a friend of mine lost his job as a full time minister of music in a church when the church reduced its full time staff, so he took a job at National Tire and Battery. I needed tires and he depended on commission sales, so I decided to buy tires from him. They have worked well in over a year and a half of driving.
Back in 1951, my dad needed to replace the tires on his 1949 Dodge. The service station that took care of his car couldn't get the 7.10 X 15 tire in the Goodyear line that it handled. Tires were a little difficult to get because of the Korean conflict. My dad bought two Davis Deluxe tires at Western Auto (this was Western Auto house brand). A couple of months later he was able to buy 2 Goodyear tires. The Davis Deluxe tires cost less and surprisingly, outlasted the Goodyear tires. I purchased tires from a store now out of business called Quality Farm and Fleet. They sold a tire called Duralon which was made by Dayton tire. These tires were much better than the Firestone 721 radials that came on my car as original equipment.
I think you will be fine if you keep the tires inflated properly and rotate them according to your owner's manual. The shop that sold you the tires should handle any tire warranty problems that arise.