Im also iPhone user but this year, i gave a try to Note10+ and it was a good experience with it. Though i sold it after using it for two months. Obviously, you can't compare A51 with Note10+ but just giving you an idea how was my experience.
Regarding WhatsApp data, no you can't transfer whatsapp data to Android from iOS or vice versa.
If you want to stick with iPhone 6s, so i'd recommend you to use google photos which gives unlimited storage so you can transfer all your photos to google photos. Plus i have turned off the whatsApp media so it doesn't save in my phone gallery automatically.
Plus, you can also buy icloud storage which is Rs.100/month for 50GB storage and Rs.300 for 200GB storage.