Could be sped up in a couple of ways. Shield of Want, CSSR +3, Symbol of Avarice and Mendicant's Staff all stack multiplicatively, resulting in 291.65% souls gained. In NG the Drakeblood Knights summoned give 4,000 souls per kill, which becomes 11,666 souls per kill with the previously mentioned items, which with your 30 second kill time means 23,332 per minute. This isn't the best farming though, as 50,000 a minute is achievable in Lothric castle on NG by stacking all the soul boosting items and using Soul Stream to kill both knights near the bonfire. ~66,000 per minute can be gained from the Grand Archives ascended winged knights on NG with all the boosts, they're also really easy to backstab and are rapportable. Souls gained can also be boosted a lot by playing in a higher NG cycle.