What steps should I consider to manage my cash flow since my residential tenant has defaulted on his rent twice (April and May)?

Defaulted since circuit breaker (April and May). I think my tenant has a good paying job, but sending a WhatsApp that he can't pay this month feels unacceptable. I offered to use deposit to offset, so that he can try to pay me the rent by December. But it would be difficult to expect he can cough out 3 months rent (2 owed, and current) in December.

At what point is it fair to have considered "nice", and looking at issuing a Writ of Distress to seize his assets for the rent defaulted?

Indeed it will become harder for your tenants to fork out all 3 month’s worth of rent all at once. Perhaps try to understand his situation? What is the main reason for him to not pay his rent when he still has his job? Is his workplace facing difficulties? As of now, perhaps you can give him another alternative instead of forcing him to pay the full rent since he might just refuse to pay. See if he is able to fork out 50-70% of the current rent every month until let’s say July when phase 2 has come into play. After that he will need to pay you back original rent fees + a percentage of what he owes you until he pays it off. Maybe use the lease duration vs how much he owe to gauge.

If he is not receptive to the idea at all, perhaps you need to look for another alternative.