This upcoming year I will be in a situation where not having Whatsapp will be extremely inconvenient. However, I deleted Whatsapp in the past because the app presents some problems for me. Is there any filter or program which would either allow someone to monitor my activity on the app, block certain parts of the app, or restrict the app so that it can only be used during the day? I have an  Iphone 5(not 5s).

This might help with preventing images downloading?

IPhone

www.techadvisor.co.uk/how-to/software/stop-whatsapp-saving-photos-on-iphone-3472339/

For Android

gadgets.ndtv.com/apps/features/whatsapp-how-to-stop-auto-download-photos-videos-songs-582469

Note that for some versions of Whatsapp you need to adjust SETTINGS > DATA USAGE > MEDIA AUTO DOWNLOAD  instead of finding the downloading of images in CHAT

Check out these apps:
offtime.co
inthemoment.io

Both should provide monitoring and restricting based on time schedules.
Hatzlacha Rabba!

Maybe the best choice is to be inconvenient.

In Judaism there's a lot inconvenience. all depends how you look at it.
It's would definitely be more convenient to drive to shul on shabbos with an air conditioned car when it's over º100 outside. let's not talk about the convenience of not going to shul at all...

But we are Jews. we know that the best thing for us is to do what's right to do even that it's sometimes inconvenient.

Many people i know gave up their smartphone completely for the reason you mentioned in your post. (and for the the sake of their children). it wasn't easy at the beginning, but they got used to it and will never go for it again.