interestingly I generally agree with your assessment of the editor until you get to font sizes.

Font sizes in HTML and make no mistake email is HTML are an exceedingly complex issue and product like Outlook send an email that consists of roughly 4 words of text and 40,000 of code to support that Microsoft office like editor it uses. Points are actually an anathema on a screen as they are a little like dividing the number of friends you have by your age when it comes to a selection method. How high is that 10 point text really. Given it's origin in putting print on paper it should come as no real surprise that is is defined has been defined loosly as since the 18th century, being between 0.18 and 0.4 millimeters. A de-facto standard exists in the graphical user interface world that a point is 1⁄72 of an inch (or exactly 0.3527 mm) so screen fonts tend to be fairly close to that. But not exactly as some screens are simply not capable of splitting a pixel in half to make 1⁄72 of an inch. SO if you compose on an apple computer with a pixel doubling retina display and the person who reads your mail is using an older android phone is is highly likely that that professionally laid out email you composed will offer the reader a very sub standard experience in trying to navigate inside it. Text may be wider that the screen and require constant left and right scrolling. The font displayed might be just about anything. Just because you have to one you selected installed does not mean your reader will.

Personally I find none of that modern or powerful. It is however a perfect recipe for failure in the hands of those that are unaware different screens make fonts measured in points different sizes and ultimately HTML may not even use the font you selected, but something from the same "font family" as I suggested before, It might be you are sending what you think is an email formatted with "Gill Sans Extrabold" as the font. What your reader may get is any font on their system that is also a sans-serif font.

I have not routinely written any email in plain text for decades using Thunderbird although I have the product configured to send a text and HTML version, just in case I encounter some of the Luddites that insist on plain text. The are still some and some of them are involved in the Thunderbird project. I am however very conservative in my use of fonts both by name and by size as be default HTML does not use points to define font sizes. See https://www.w3schools.com/css/css_font_size.asp there you will see that fonts in HTML as defined largely using words or in more extreme cases as pixels

px: Pixels offers fixed and precise control over the font size. xx-small, x-small, small, medium, large, x-large, xx-large. These keywords has a predefined set of sizes in browsers.

As you have no real idea just how big a pixel is on your readers display you take a gamble specifying that in size (although Thunderbird does allow it) what is far more reliable as the basic sizes normal medium etc, hence Thunderbird's font selector offers to make fonts bigger and small not specifying a particular point size as size as 1⁄72 of an inch has little meaning on a 4" display. What does have meaning is that the local reader will use a built in zoom approach to make normal sized text readable and heading sizes a larger size again based on it's internal metrics as to what is readable. It will make not attempt to even look at your 11 point font and will strictly render your 13 pixel font over 13 pixels regardless of the real world size so expect complaints from folk about being send micro dots or worse still clownishly large text. I see complaints here in this forum over the years from folk that think writing an email is the same as writing a letter on a typewriter,. The sort answer is it is not very similar at all ASo to your original question.

When will Thunderbird provide powerful and state of the art formating stuff?

Possibly never, as an open source project there will undoubtedly be improvements to the editor component but they will be temptered by the community and the imperative that email is a communication medium. change will probably be slow and come in starts. The editor is dated and long over due for an overhaul, but will you ever get you word processor? Probably not, as it will forever be tied to offering a good experience in both composition and reading of email. You can not offer the level of control to the sender you ask for (exact sizing to the pixel and specifying font names to be used) and try and guarantee a similar level of reading experience. Microsoft do it with their emails, at the expense of composing huge and complicated emails that often only work correctly in Microsoft mail clients because they have "innovated" and at times requiring the download of a font larger than the email to read it with. That is something folk paying for expensive bandwidth in some countries object to (that can sometimes be the ones insisting on plain text as is get the message across without the expensive overheads of "flair")

2) Is there an addon which adds powerful and state of the art formating stuff?

Have you looked? Seriously I am happy with my lot and have not but a quick search indicates the first on this list might do the job. https://addons.thunderbird.net/en-US/thunderbird/search/?q=font&appver=9999.0

Uff, a lot of tech talk. I'm used to write whole book in Word or creating slides in Powerpoint and it is totally normal to use more than 6 different font heights (tiny - huge). What, if I like to use huge+ or super huge? What will come next after superhuge? I'm technician working with CAD Software since 30 years and yes we are using different heights everywhere. So, your answer sounds like a big excuse. Other companies can easily do it.

But there are other problems than only the text size. Writing something pressing enter causes every time a paragraph. So I need to use Shift + Enter. Adding a table causes all time a paragraph space around it. Using bullet points will cause the same. Deleting a table is not possible or the feature is very good hidden. Adding a text after a table, if there is nothing, is not possible or very good hidden. Using a different color for a word needs 5 clicks. Inside Outlook, Word, Powerpoint, .... all programs nowadays need 2 clicks.

I guess there are again 1 million reasons why these wishes are technically a mess, but again, this is state of the art.

Actually I try to reactivate an outdated addon to fulfil my wishes.