I believe the most famous places for hydrangeas in Kamakura are Hasedera and Meigetsuin. I have been to both at the very beginning of the hydrangea season, and they were nice but arguably not as impressive as some places I've been to in other parts of Japan (at least as far as hydrangeas go; both were very nice as temples and outdoor places). The problem with both of those places is that they become insanely crowded at the height of the season. I mean really insane, like having to wait in line for very long periods and then when you finally get in still being stuck in a long queue just to view the flowers. At Hasedera I believe that at least on weekends they have some sort of "take a number" system to minimize the chaos, but you are still guaranteed long waits. Now, this is in an "ordinary" year, and particularly on weekends (but weekdays are also bad; I went to Meigetsuin early on a weekday morning at height of the season last year and there were several hundred people waiting in line for the gate to open already 30 minutes beforehand, so I opted instead to visit some nearby temples that aren't famous for hydrangeas and had a wonderful time). I'm sure things will be much less crowded this year because of the coronavirus, but you still might want to think twice about going to Kamakura if your objective is to see a lot of hydrangeas. Even if the number of viewers is only half of the usual, it will probably still be crowded. (Do you want to be in a crowded place like that yourself? Trust me, it is not a serene nature experience. Viruses aside, the people are noisy and annoying.) I will say, if you do go, then make sure it isn't a weekend. But possibly so many people will be scared away that you won't have any problems. It could be an historic best year for going to these places. The hydrangeas don't give a hoot about the virus and will be blooming to beat the band.
If you want to see nice temples and almost certainly at least some hydrangeas, then Kamakura is a good place to go. If it were me, I would just avoid Hasedera and Meigetsuin. There are loads of other good temples there.
There are lots of other places to see hydrangeas in the Tokyo metropolitan area. If you aren't looking for massive plantings, you will see them in many parks, gardens, and temple grounds, as well as simply planted for landscaping along railroad track embankments and other places.
Two places I've enjoyed seeing hydrangeas in Tokyo without serious crowds are at hydrangea festivals at Toshimaen and Hakusan Jinja. I'm not sure what the status of Toshimaen is this year (the amusement park will close at some point in the near future but I can't remember when, and then I'm not sure what they will do with the hydrangea garden/festival). Both festivals could possibly be cancelled because of the virus, however. Depends on how bad it gets and how worried people are.
Here is a Japanese page from last year that identifies some recommended hydrangea viewing spots in the Tokyo area, but many of them are not actually in Tokyo and are hard to get to. You might find a few possibilities by using a translator. https://www.enjoytokyo.jp/style/110647/?__ngt__=TT1060e9bac003ac1e4a59...
The first on the list is actually Ueno Park by Shinobazu Pond (probably not because of the abundance of hydrangeas but I suppose because it is so central). Toshimaen and Hakusan Jinja are #3 and #5 on the list. But again, you should verify things before you go to any of these places. A hotel concierge or a good tourist information center should be able to help you by calling places to make sure they are open. As you get closer to June there will be more information available online on places to see hydrangeas in 2020 (google for it), and tourist information centers should be able to make suggestions as well. (I have never had much luck with hotel concierges as far as getting advice for nature experiences.)
If your trip includes Kyoto, there are a lot more places to see hydrangeas there, and many of them are not crowded.