So what, you keep an ungoogled-chromium around and use it occasionally for compatibility, if you really need to. Doesn’t mean you are obligated to use it as your daily driver.
WebRender has been enabled by default on Firefox for Android since version 92 (September 2021). Performance is fine for me, especially with uBlock Origin.
Performance is fine for you since your hardware is decent. The performance issues becomes more evident on slower hardware, like my 5 year old Galaxy Tab S5e. It's laggy, stuttery and slow to load pages, while using s chromium based browser works mostly fine. UBo isn't a reason to stick with Firefox either, the Kiwi Browser has extension support
uBlock Origin works best on Firefox, according to the extension author. This is definitely a reason to use Firefox, and even more so when Chromium phases out Manifest V2 completely.
Your Samsung Galaxy Tab S5e has a PassMark score of 5488. I've been happy with the performance of Firefox on a Google Pixel 3a, which has a nearly identical PassMark score of 5483. Maybe your expectations or experiences are different, but I'm comfortable with saying that Firefox's performance is fine for me on a range of Android devices, old and new.
What if you need to use their site, e.g. to file taxes, or apply for university? There are unfortunately cases, where you have essentially no choice, but to install a Chromium based browser
Try and register something on Razer's website so you can get support for it. Until a few weeks back, PSN locked the browser up completely when signing in.
I use FF for day to day use, but prepare to swap to Chrome when things go wrong just because most sites aren't tested on it.