OK, this is where drawing user flows comes handy.
See, in your scenario you're assuming the user will read the welcome mail on a mobile device. But... what if the user opens the mail on a desktop or laptop device? How will you sniff anything? What if the user has 2 mobile devices with different OS but only one of those is the preferred one? Just in case, I didn't even need to think long on this: it's exactly my case. I never, EVER check mail on mobile, only desktop, laptop or iPad. And I have several mobile devices, 2 of them I use primarily, only one of them is the preferred one.
You may say
"OK, but most people actually DO check their mails on mobile"
, which may actually be correct for most demographics. Then.... why don't you use this for your advantage? Instead of all those steps, sending user to some web page to maybe discover that she is using the wrong device then abandon journey is the only way to go, simply do this:
- Send mail. Body of mail includes "Download on Apple Store" link (then Android, Windows, whatever). Most users will know that apps need to be downloaded from a mobile (also tablets, but this will depend on your app)
- user installs app and logs with mail address, choosing password on first use
- That was really it
This way, there are only 2 simple steps, no ambiguity on OS or friction on what to do, it really is as simple as this.
Alternatively, you can use a different approach: instead of sending mails, send messages using mobile only apps (such as whatsapp) That way you'll also avoid people opening the welcome message on a desktop or laptop computer.
Please note that your app and the target for your app are of paramount importance. There are lots of studies about messaging usage based on age, country and what not. For example, on most countries, if your app is for young audiences, just ditch the whole mail approach and go for a messaging app or social media login.
These different demographic preferences will tell you a lot more than you may imagine and you need to be very sure about them since they will accurately guide your steps