I’m in the same situation like you.
I try sticking to standards, paying attention to the compatibility notes on MDN and hoping for the UI frameworks I use to care for cross-browser consistency.
There are paid solutions to test on different browser like https://browserstack.com, but I haven’t used any of them.
Sometimes I ask coworkers to test my web apps on their iPhones with Safari to see if there are any funny things going on.
Task 1
I heard using line-height on inputs tend to get more consistent results. I have to admit that I haven’t done extensive testing to see where the actual differences are.
I recommend also adding -webkit-appearance: none; since only the newest Safari supports it without the prefix.
For the icon it may be a bit easier to use it as a background image on the div, add some width and height and change the form to Flexbox with align-items: center;
Task 2
This looks very good. You could add some width to the labels (larger than the content) to make the radio buttons align nicely.
Task 3
Very cool work on the icons and colors. Nothing to complain from my side.