Just to add to the fray, if you don't have problem for parking two cars,
and money is not too much of an issue, I would keep the Miata.
The Model Y is certainly nice to drive in many aspects,
but it handles more like an SUV compare to the Miata.
If you need to drive in a city where street parking is difficult,
having a much larger car might be an issue.
The turning circle of the Model 3/Y is not great,
so you will have to perform a 'K' turn in many situations
instead of a simple 'U' turn.
This makes also parallel parking even more difficult in a tight spot,
as you will need to perform many maneuvers compare to the Miata.
Otherwise, the Model Y would provides a lot more space and
the motor will be even more powerful compared to the Miata.
I noticed that some user mentioned getting the Model 3.
The driving would be certainly nicer but if you have a dog,
you will have to use the rear seats.
There are some special padding to do cover the rear seats.
Or to have the rear seat folded, so your dog will have plenty of space,
but you will lose your trunk space.
I was in your situation, I had an older VW GTI, and I finally kept it,
as I was able to park both cars.
Having a second car don't cost to much extra insurance,
especially as I insured it for a low mileage.
In fact, my Tesla was rear ended, and it took about 9 months
to get it repaired. So having a second car was very helpful.
Otherwise, having two cars might not not really be justified.
Also, ICE cars are not a good investment, as the sale of them
will be ban in the future, and will be even more difficult to sell.