Confesso que não conheço os sites dessas marcas, já há muito tempo que não pego num Samsung. Se forem referenciados por users com boa reputação no XDA deve ser minimamente confiável.

És capaz de encontrar uns telemóveis em segunda mão baratos, verifica que têm a compatibilidade que queres. Penso que o OnePlus 6, por exemplo, funciona bem com o postmarketOS.

Para voltar ao firmware inicial tens de ter o ficheiro que o contém. Para os Samsungs isso deve ser fácil de encontrar, e depois podes usar o Odin para fazer flash (não é um processo muito complicado que me lembre). Para o Alcatel, tenho as minhas dúvidas.

Quanto a dual boot, esquece isso, se o suporte para o resto das coisas já é o que é, não deve existir nada que te deixe fazer dual boot. Infelizmente os telemóveis não funcionam como computadores :(

Se usas um destes devices regularmente, então talvez seja melhor não tentares brincar com ele, há sempre o risco de dares brick permanentemente neste processo, especialmente se for a primeira vez e estiveres a usar ROMs não muito oficiais.

Para Linux, recomendo PostmarketOS, embora penso que o suporte para esses dispositivos não seja muito bom (ver as páginas abaixo):
https://wiki.postmarketos.org/wiki/Samsung_Galaxy_S_II_(samsung-i9100g) (não parece ser exatamente o mesmo modelo)
https://wiki.postmarketos.org/wiki/Samsung_Galaxy_Tab_A_2016_(SM-T580)
Páginas para o SM-T280 e 4047D não parecem existir

Quanto a LineageOS, o processo é geralmente simples, embora seja provável que a maioria do que encontres esteja desatualizado. Além da wiki deles, recomendo procurares no XDA Forums, sempre tens mais algumas opções de ROMs.

Se tiveres mais alguma dúvida diz!

I've been using email aliases for a few years now, but all spam I get is addressed to my main email (which admittedly is readily available on my website). Seems like no one has sold my email address yet

While the disposable cards might not always work, you can always create a regular virtual card and delete/freeze it afterwards. Not sure if there are any limits tho

Not really the same thing, but Revolut has disposable virtual cards that you can use (but it's a full-fledged bank).

Some apps like MBWay in Portugal also allow you to create these virtual cards, but it requires a Portuguese bank account.

I thought it was a problem with my phone since I'm using a custom ROM and it did not happen before. When I open Firefox and it has been in the background for a while, it shows a black screen where the web content should be and often crashes if you try to open another tab or do something else. Also happens if I open a link from another app. The only solution is to close Firefox and swipe it off the recent apps and reopen it. Is this the same problem you have?

Seems to still be available on F-Droid, but the last update was in 2017. I've installed it, but ended up not logging in since it's targeting an SDK version so old that Android is giving me a bunch of warnings (and it doesn't even scale to my entire screen, there's a big black space at the bottom)

I've had this "problem" before, and I did not want to use the full Messenger app for privacy reasons as well. I had installed Messenger Lite, but it was discontinued a few weeks ago. I now have Facebook Lite, which also has Messenger, and you get notifications as well. It's not perfect, but don't give it too many permissions and you should be mostly fine. Using it in the browser is absolutely terrible, and you would not get push notifications.

That's great to hear! There are still some apps on my phone that aren't taking advantage of it then -- or perhaps it has something to do with my custom ROM.

I'm unsure if this is still the case in the latest Android versions, but the terrible camera APIs in 3rd party apps/social media apps is still a big reason why my non-techy friends switched to an iPhone. Even if the quality of pictures using the stock camera app is good, if the photos that come out while using Instagram or Snapchat or whatever kids are using these days are low-quality, they'll consider moving to an iPhone. Again, I'm unsure if the APIs already exist or if it's just app developers that are not using them. Either way, there needs to be a push in this direction.

I personally use Firefly-III. It is completely self-hosted (there is no cloud version that I know of), and is quite feature-complete. Their data importer allows you to import transactions directly from your bank (check if you bank is supported, but all European banks are), so you don't need to create anything manually.

For example, I have the data importer running daily, and then I only have to set the correct category for each transaction, which doesn't take that long.