Can I make a paid app on playstore free on fdroid ?

Sure , indeed there are some foss app that are the same as you described

Yes, a lot of developers have done this. Many examples have been posted on this thread (OsmAnd, Conversations, Davx5) - Mindustry is another example. free on f-droid (and Google store too I think), but $10 on Steam.

If you own the copyright then yes this is 100% legal.

There are already apps that are like this. They usually add a couple features to the paid release so that people feel like they are getting something extra for the money. The good ones will eventually move those features to the open release eventually. However, this incentivizes keeping part of the app closed source so that nobody can just rename and re-release the paid version.

It is 100% up to you for how to handle these tradeoffs. Personally, I think so long as you are principled and ready for some criticism - and can handle it gracefully - getting paid for work that builds your open source app is a very good idea. We don't all have the luxury of maintaining high quality unpaid side projects!

Conversations, an XMPP chat app, does exactly this.

Free software is a matter of liberty, not price. It is perfectly legal and ethical to sell free software. Keep in mind if you're using third party code (whether it's libraries or external contributions to your application) you must abide by the terms of whatever license it is under, this is whether it's paid or gratis.

It's even perfectly legal to fork an existing free software project and sell it on the play store, although whether that is ethical or not is up for debate - depending on what efforts you put into your fork before selling it, an orthodox Stallmanist might have no problem with it but the original developer(s) of that code may perceive this as "theft." Keep in mind you must abide by the terms of whatever license the project is under, so if it is a copyleft license like the GNU GPL you must either provide corresponding source code or an offer for such.

Some just do it to make some extra money and save for convenience. Krita is free to download and install yourself. It's 10 bucks if you want to get it on Steam. It's perfectly within their right. Especially in KDE's instance.

Is this your app? If yes then sure, it’s your app you can do whatever you want with it. If you are still morally troubled then just offer a free apk for those who don’t want to pay but also don’t want fdroid (for whatever reason there is, I don’t know of any).

As long as you are the legal owner of the code, I don't see why you couldn't.

For example, DAVx⁵ (F-Droid, Google Play)

I'd see it as a tradeoff between the convenience of silent updates of Google Play and incentivizing people to go for F-Droid.

I can think of a few apps that do this. It is certainly possible. I think it is ethical; if someone is not participating in the open source community, they miss out on the benefits. I think most people involved do contribute in some way. If someone just wants to use Google for the benefit of ease and discoverability, then they can pay for it. You're still offering an ad-free app (presumably) and adding use value. It's perfectly reasonable to suppliment the cost of development in is way.

Why would it be illegal, or unethical? I don't really see any reason.