The volume problem, stated plainly
The demand
A transitional reset does not arrive client by client. It arrives for every private business at once, on the same date, with the same evidentiary requirement. A firm with three hundred private-business clients has three hundred valuation questions landing in a single window, and a signature is not something that scales by working later.
The alternatives are all bad
Sign them yourself and you carry the independence problem and the professional risk. Send them to a Big 4 firm and most clients can't justify the fee. Leave them to the free apportionment method and the ones with uneven growth curves quietly pay more tax than they needed to, and you will hear about it later.