As for your firefox issues, thats pretty trivial to fix by adding some CSS in usercontent.css/userchrome.css

See here for some examples: MrOtherGuy/firefox-csshacks

Firefox’ tabs are MASSIVE

You can change the tab size.

Go to about:config and change browser.tabs.tabMinWidth

The answer is Firefox CSS: https://firefoxcss-store.github.io/

You can change firefox' start page with addons.

Iceraven is a fully open source Firefox fork.

Ungoogled Chromium is the same for chromium.

you can change how firefox looks by modifying userchrome.css and usercontent.css files easily, if the issue is how it looks maybe you can find a theme you like

for example here is a video explaining how to modify the start page backgrounds using css files on firefox here

Brave does support opening tabs from other devices, sync works good so long as it always has at least 1 device in the sync chain, so if you only have 1 device and have to reinstall it the settings might be lost, but if you have 2 devices and reinstall one the settings are still saved whenever you rejoin the chain. The reason is there are no accounts saved in brave, so the only way to ID your browser is by the sync chain. If the sync chain has no devices it may be removed from the sync servers.

All of the crypto rewards stuff can be disabled with 1 switch, and a second switch if you also want to turn off wallet, but it's not really active unless you configure it. Rewards is there as a way for them to make money without having to make Google or Bing the default search engine as other browsers do.

Brave is a great browser, but Firefox is also great and very configurable. And thanks to this thread I learned that FF's interface can be customized, which was one of my main reasons not to use it anymore. I'll play with it again, it's important to have a non-chromium based browser as an alternative.

You might want to look into Firefox forks—LibreWolf, maybe? Or Seamonkey, which is technically not a Firefox fork but might as well be. I use Pale Moon, but while it does have small tabs and no data collection that I'm aware of (even the code supporting it may have been ripped out in the ongoing efforts to get rid of unwanted maintenanace burdens), it uses the old Firefox extension framework that was deprecated some years ago, so you'd likely have to find alternatives to some extensions.

For the small tabs, you may try to enable custom stylesheet (userChrome), there are plenty of themes on the web, but if you're looking for something more specific you may edit it yourself, it's very easy. Oh and the default start page is pretty ugly I agree, there are plenty of alternative start pages on the add-on store For the data collection, firefox forks like librewolf should have that disabled by default (I don't have any experience tho)

While it may be possible, the more requirements make it harder to meet.

When you move from one relationship to another, you don't expect your new partner to be exactly like the old one. You accept and learn to love the differences.

Stat page and tab size are very particular deal breakers. Are they really key? I prefer FF tabs to Chrome as I can see what I'm clicking on and not accidentally close it. I don't stay on home page long enough to notice. My old tabs open at start up.

I use the Tabliss extension to change my start page, but there's other ones as well. If you use UBlockOrigin or Privacy Badger those have counters for the amounts of ads and trackers blocked.