Performance of Samsung M21 2021 4/64 version will depend on the usage of the phone I think. The phone might not give good performance for a heavy user, who likes to install lots of apps, and plays games and uses Facebook, Instagram... memory hogging apps, etc.
For general usage, like one who likes to use Whatsapp, YouTube and such basic apps, I think the phone is a decent option. I did feel a very slight lag when more apps were installed on the phone, and while updating the apps on Play Store. But, that was noticeable to me, because I notice such things. A general user won't notice. It has been some days while my friend has been using the phone and he is satisfied and happy so far.
I will be keeping an eye on its performance whenever I get to see that phone. Considering the weight of Moto phones in that range, and also the irregularity of updates with Moto, Samsung phone does seem like a decent option to consider.
But yea, with further updates, don't know how the phone will behave. Will have to keep an eye on that.
Regarding bloatware, it is a mix. I was able to uninstall several apps, including those from Samsung, but some I were unable to uninstall and just had to disable them. But yea, bloatware is a big problem on Samsung phones, and one of the reasons for it being slow. I particularly hate the Samsung Store app, which is useless when already there is Google Play Store with everything you need.