Continue with nanny or transition to daycare?

We have a 19 month old who has been with our nanny since he was 3 months old. They get along great and she clearly adores him, and provides excellent care. It has also been so nice to have him home (I WFH) when he was younger and breastfeeding. Our nanny also takes care of a lot of child-related care, like doing his laundry, prepping his food etc, which is super helpful for two parents who work at least 40 hrs/wk.

But a spot at a very good daycare just opened up and he could start in 4 weeks. We never told the nanny that we were in daycare waitlists because this was the first list we put ourselves on and we thought the wait would be at least 3 months and would have time to let her know.

The daycare is also about half as expensive as our nanny. So that is a major factor in the decision. We have been making the nanny work but it is a significant strain on our budget.

So, should we bite the bullet and move him to the daycare? Or just hold on to the nanny for a bit longer, until another spot opens up in 3-6 months?

If we move him into daycare, should we do part time for the first month or so to help the transition? Cost would be about flat for us that month. I am terrified of the cycle of daycare illnesses and am thinking I could pay guaranteed hours (20 hrs/wk) during the first month.