My personal experience: 121 chats (just my family account I have several other accounts)
Spam from the same address will appear as a single chat, and you can block the contact and never receive more spam from that address
Since spammers constantly change address you might what to tweak the "show classic email" option in Advanced settings, if you set it to "accepted contacts only" only classic email from contacts you have accepted a chat already will show up in Delta Chat so contact requests from new contacts will no show up, which is perfect if you only want your dad's messages to appear in the app
Your parents can use something else to message you, for other people they can switch to SMS in worse case, just uninstall the app with a goodbye message once you are not there there will be some real reasons for other people to join you elsewhere,
There was a time where there was no WhatsApp and no one died because of it, true there might be some extra inconveniences but the question is if you have the will to do the change, I deleted WhatsApp and things are OK so far I am not using Signal either, I use Delta Chat and email mainly
I use Delta Chat exclusively to chat with my family, I don't even have WhatsApp installed.
Delta Chat has most of the features you need for communication the only missing thing is integrated calls that cause the phone to ring, which is not a deal breaker for me since I don't call often, used the jitsi meet integration once with them was a bit messy to get my mom etc to figure out how to use jitsi meet in the browser while explaining via voice messages in Delta Chat, but I think adding integrated calls is in the roadmap
That is a "they" issue, they don't allow to use their email server with your own email client of your choice which sucks, better use other email service. You don't need to use your email account tho, you can register without providing any information and it will automatically create a random anonymous email account for you, much better than the protonmail one, for free and without sending limits and with blazing FAST instant delivery
I would say this is the normal behavior and without this apps wouldn't work properly or connect in background, ex. to receive messages in chat apps and display a notification ex. when the system wakes them up via push notifications