I would try
Natchez, Ouachita, Ponca and/or PrimeArk.
I have Ouachita and have tried most of the varieties from the University of Arkansas breeding program.
https://aaes.uada.edu/fruit-breeding/blackberries/
They have a great series of thornless standing/erect cane varieties that are great producers and can be managed for commercial production. Also the most cold and heat resistant.
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University of Oregon is the other leading breeding program in the country.
https://agsci.oregonstate.edu/berrycrops/breeding
These are trailing types and aren't that hardy, but are one of the earliest fruiting varieties.
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Most fruit on last years growth, so you need them to "survive" the dormant period. I believe Chester (low thorn) and Triple Crown (thornless) are also excellent, and you might try Marion berries if you can, but these are all later fruiting.
If you have trouble overwintering, you should especially consider PrimeArk and the U of A series that produces on first year canes (primocane varieties).
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Apache: ‘Apache’ is the latest ripening thornless Arkansas variety (~June 25). It is high yielding and has large berry size. It has excellent postharvest potential and is good for shipping. Flavor for ‘Apache’ is considered good (sweet), between ‘Navaho’ and ‘Arapaho.’ The chilling requirement for ‘Apache’ is about 800 hours, one of the higher totals among these varieties along with ‘Navaho’
Arapaho – ‘Arapaho’ is the earliest ripening thornless variety (~June 7). It yields lower than ‘Navaho,’ ‘Apache,’ or ‘Ouachita.’ Fruit storage and handling is good. ‘Arapaho’ has a sweet flavor. The chilling requirement for ‘Arapaho’ is around 500 hours.
Chickasaw –
thorny
Kiowa –
thorny
Natchez – ‘Natchez’ is an early thornless, semi-erect blackberry that ripens around the same time as ‘Arapaho’ (~June 3). It has large berries (8 to 9 grams), comparable to ‘Apache.’ Yields are comparable to ‘Ouachita’ and ‘Apache,’ and twice that of ‘Arapaho.’ Flavor is good, similar to ‘Arapaho.’ Fruit stores well, so shipping is a possibility. Chilling requirement is likely around 500 hours.
Navaho – an erect, thornless blackberry. The fruit are large and firm, and are less tart than other thornless varieties. They ripen late in the season (~June 20). Only about 65% of root cuttings produce plants, so they are more expensive than most thorned varieties. This variety is moderately resistant to anthracnose, and is known hardy to -14 F in Arkansas.
Ouachita–‘Ouachita’ ripens between ‘Arapaho’ and ‘Navaho’ and produces very high yields, among the highest for any Arkansas thornless variety (~June 12). Berry size is larger than ‘Arapaho’ and ‘Navaho,’ but not as large as ‘Apache.’ Postharvest handling is excellent, comparable to ‘Navaho.’ ‘Ouachita’ also has very erect canes. It may have a lower chilling requirement than other Arkansas thornless cultivars, possibly 200-300 hours.
