ATmega48PB: wdt in interrupt mode still reset the MCU
If you program the WDTON fuse (ie, set it to the non-default value of 0), then according to the data sheet you are locked into System Reset rather than Interrupt mode; this would not appear to be what you want.
It would appear that what you do want is a combination of Interrupt and System Reset mode, where the first count expiration sets WDIF and triggers and interrupt, and the second causes a reset.
However, if you do that without an ISR, you're going to need to be sure to keep re-setting the WDIE bit after it is cleared on each counter expiration, as if the count expires when WDIE is already unset, a reset will occur.