A far as Srardh is concerned, we can neither postpone or advance as it will attract strong Pitru Dosha. The best way is to perform Srardh in a simple way as Hiranya Srardh with only 2 priests without Homa, cooking or other eloborate arrangements on the same (appointed) day. Care should be taken to bring the priests with approvals /allowed Timings as per rules, properly sanitized and with other health precautions.
If even this is not possible in extreme cases of lock down like in war-times, scriptures say only during such war-times or critical periods, one can do Srardh during Krishna paksha (after full moon) Ekadashi or Astami or Amavasya (new moon day). But when postponed, the person should observe full and strict Upavas (only with fruits and water) on the actual Sradh day (on which day he could not do Srardh). The best option will be however to do at least Hiranya Srardh on the same day to get full blessings of Pithrus.
However, rituals of ceremonies like grahapravesam, weddings birthdays, wedding days, reception etc have to be / can be comfortably postponed.
Seemantham or Pumsavana Seemantham, also known as Valaikaappu, celebrated normally during the 6th, 7th or 8th month of a woman’s pregnancy can also be performed simply with a priest or two without much grandness or elaboration during lockdown following strict rules and guidelines and only with appropriate permissions. Although in ancient times Seemantham was performed on the birth of each child, today it may be limited to the first born. This ritual is similar to a baby shower performed in Western cultures.
All other rituals can be postponed.
Hope it is clear.