Seventh Day Baptists are Baptists who observe the Sabbath, the seventh day of the week, as a holy day to the Lord. They adopt a covenant Baptist theology, based on the concept of regenerated society, conscious baptism of believers by immersion, congregational government and the scriptural basis of opinion and practice.
So, as you say, there seems to be no other differences between SDB & 'regular' Baptists other than the 7th day Sabbath observance - the S of F is Bible based. It is only if they were to make keeping the 7th day Sabbath a salvific issue, which is contra to what Paul taught -
Romans 14:5-6 One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks.
Colossians 2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: