I would personally go with the pixel line, as they just have the google spyware, and not 3 other companies spyware installed on top of Google spyware. Also they generally have decent security updates, and get them faster than most other brands, and then you have the option to switch to grapheneOS should you want too. They are also fairly affordable especially if you are getting them second hand.

I'd say Fairphone is less worse than e.g. Samsung or other big companies from a privacy perspective. Their phones are also promised long support cycle, meaning software updates as well as replacement parts, so buying used should be more viable than other brands. Although I believe they might be difficult to find if you're not in Europe.

I highly recommend you use GrapheneOS with Sandboxed Google Play Services, nearly perfect compatibility and much more private and secure than any stock Android

Have you tried cloning the app and making adjustments to it? https://appcloner.app/

Are there better brands?

Probably. I'll echo the thoughts and opinions of others, sorry for the repeat.

Pixel phones will continue to receive updates (and GrapheneOS supports Pixels as long as Google supports them). If you go full tinfoil hat, you'll see that there are people out there accusing Google of being an NSA partner and more.

Fairphone is a neat idea for reparability reasons. They also try to support their devices for a long time. But, they can be expensive for less than premium hardware.

Try to find devices that ship with very little bloatware:

• Nokia makes some cheap phones that might be tempting, but HMD Global (licence-holder for Nokia) has messed stuff up in the past. My Nokia 5.3, for example, received a bad update. What was bad? Well, if you tried to factory reset it bricked the phone. I did not read the forum before resetting my phone (https://community.phones.nokia.com/discussion/66312/warning-do-not-factory-reset-nokia-5-3-bricked-phone) so that cheap little guy didn't even make it to year 2. Probably for the best as they were once accused of sharing data with [2 very large countries].
• Motorola, like Nokia, makes some cheaper devices. And, like Nokia, tends to ship with little to no bloatware. But, they are notorious for abandoning devices.
• Sony and Asus does not fall under the cheap category, unless you buy used.

If you really want to save, buy a broken phone and fix it. Rounded.com sells spare parts for different phones, so you could even find a phone for cheap and make it last using spare parts.

If you are rooted and the app doesn't need any gapps, try hide prop config magisk module. (Some apps have fallbacks for gapps built in like Uber has OSM as fallback for gmaps in India atleast)

Fairphone.