I've used AirVPN for over a year now. No complaints. Mullvad stopped port forwarding so had to swap. Recently moved email to proton so might move VPN over soon too.

AzireVPN is the best in my opinion

Centralization and monoculture is a mistake.

One thing to keep in mind as new is that "VPN" is a technical term with pretty clear meaning among the technical people but it has a very fuzzy meaning in marketing and branding. Referring here to "VPN apps" that may just be a local DNS relay (ie: it will only tunnel and filter your DNS requests; all your actual traffic still goes through your normal connection as clear as always). Oftentimes, it's what we would call a proxy. Android has not at all helped here.

In either case, yes, you can usually chain things. What if any benefits you get from that depends on both technical specifics (which protocols) and your circumstances and threat model.

For example, if we consider only Wireguard (one of the VPN protocols Mullvad offers).

No VPN/proxy: Your ISP sees everything

1 proxy: ISP sees that you are connecting to proxy but not what servers you're actually talking to. VPN provider now sees everything instead.

2 proxies: Proxy A sees your encrypted traffic to Proxy B. Proxy B sees all your traffic but doesn't know where you are.

3 proxies: Congratulations, you have manually built a shitty onion circuit (Tor works like this)

Mullvad has their own "multi-hop" feature which chains two Mullvad nodes but i have to question using that strictly for privacy reasons, considering it's by the same provider and the ports make it predictable from the ISP.

Mullvad VPN works well on Android and has some DNS based ad blockers & content filters in the VPN app (though off by default iirc). Mullvad browser is not ported to Android.

That said, it's important to understand that VPNs don't provide privacy in any absolute sense. They can (maybe) obscure data about your browsing habits from your ISP. But they won't stop all the other, more effective tracking exists nearly everywhere else on the web.

You can combine VPN and DNS-based ad blocking, usually. Mullvad has it's own dns server with ad filter. However you can use any other.

I don't think Android supports two different VPNs.

Proton includes an ad blocker built in

Mullvad has an option in the settings to enable their own DNS filters.

In case it swings your judgement either way, Njalla is run by one of the three Piratebay founders.

Acquired by a shady company: https://en.wikipedia.org/wiki/Private_Internet_Access

PIA is Israeli now (Kape Technologies, a malware distributor).