Car seat issues

My 5 month will only stay in the car seat maybe 30 min it’s a soft car seat made by Britax but baby also will try to sit up in the seat and get very upset when realize can’t sit up in it. Is it time to go to a convertible car seat? Is babies just not meant to be in the car seat longer then 30min? Idk what else to do but it’s getting to be annoying and stressful that if I take a 20 min drive spend 10 min in the store by time we driving home baby trying get out the seat and I know it’s not hungry because I never leave my house until baby has had a full bottle and once bottle is done baby is burped I’m out the door so ik baby isn’t back hungry in 30 min I’m just tired of felling stuck in house because kid won’t stay in car seat. Any brands that are good?