Strictly in accordance with governing code which specify the need for PWHT, no welding is allowed after PWHT. Else PWHT will nave to be repeated to relieve any residual stresses that may have been introduced by the new welds.
You may opt however for a localized PWHT if cost/time is your consideration to avoiding repeat PWHT.
you may consider also the use of welding procedure, qualified with CTOD (not really sure about this, as normally it is used to obviate/waive the need for PWHT on thick materials)
https://www.twi-global.com/technical-knowledge/job-knowledge/ctod-testing-076. And I reckon this tank is relatively thin-walled to buckle after draining. If I am the client I might consider already requiring a repeat PWHT due to the jacking operation on the buckle.( or worst, scrap the tank)
my 2 cents