i recommend only installing work comms (email, slack notifications) on work-specific devices (computer, phone), never on your personal devices. Then turn all work devices off except for during the contractually agreed hours when you are paid to work.
If you have agreed to do an on call rotation then you need to be contactable, but you should only be doing an on call rotation if you've agreed to do it and are getting paid an additional amount in return etc, not as some extra new unpaid duty they try to spring on you in addition to your regular duties!
Instead of framing it as "laziness" you could try to reframe it as "business" (i.e. your own business where you exchange your time and ability to perform software development services in exchange for money). Your client -- the people who run the startup you are providing services to -- are presumably business people, they should be used to negotiating with other business people. If they were to do a bunch of work for their own clients for free without negotiating any payment, that would reflect poorly on their abilities as business people. Similarly for you.
You don't have to demand anything, demanding is generally not a helpful way to frame things in commercial negotiations -- a more accurate way to frame things is "request". But you can set and then communicate your boundaries clearly. Your employer cannot force you to be available on chat outside of paid working hours, so just stop doing it and tell them that you're not available for unpaid overtime. No need to demand, just do, then communicate.
Maybe words along the lines of: "i understand $NEWLY_LAUNCHED_PRODUCT will sometimes need support outside of business hours. the current contract doesn't include a provision to pay overtime if i'm on call for out of hours support, so unfortunately for business reasons it doesn't make commercial sense for me to provide free out of hours support. i'm open to helping $STARTUP with $PUTTING_OUT_NEWPRODUCT_FIRES_AT_3AM_ON_A_SUNDAY, but we would need to update the contract to reflect that and get a commercial arrangement in place that's win-win for both parties.