Eventually I gave up on trying to use $\LaTeX$'y compliant escape sequences for every non-ASCII character, and decided to solely switch to XeLaTeX for scientific writing with a set of fonts which support as many characters as possible (UTF-8 only documents).

That's said, if you are on Windows, I can recommend to use a free program called WinCompose to quickly insert practically any special character one can think of (of course, if the font has its symbol implemented). This works in every text field where Unicode is supported, with MathJax too.

Basically the program adds an extra layer for the user allowing to use intuitive keys combinations instead of dull Unicode codes which are hard to remember. Pressing a Meta key initiates the input, and if it matches a program defined macros, a corresponding symbol will be inserted (Meta is typically an Alt key, by default right Alt); here are few examples which I personally cannot live without (I use US keyboard layout):

Meta o A = Å
Meta % o = ‰
Meta % % = ‱

Meta " a = ä
Meta s s = ß

Meta * l = λ
Meta * L = Λ

Meta / = = ≠
Meta + - = ±
Meta Meta o i i i n t = ∰

Meta o o = °
Meta E = = €
Meta o R = ®

Meta < - > = ↔
Meta \ SPC = ␣
etc.

This is by no means a complete substituent, but rather a nifty tool which makes work with special characters less miserable and calling for a Character Map less frequent.

Eventually I gave up on trying to use $\mathrm\LaTeX$ escape sequences for every non-ASCII character, and decided to solely switch to XeLaTeX for scientific writing with a set of fonts which support as many characters as possible (UTF-8 only documents).

That's said, if you are on a desktop computer, I'd recommend to invest some time in getting to know a compose key to quickly insert practically any special character one can think of (of course, if the current font includes this glyph). This works in every text editor with Unicode support and with MathJax in browser, too. Most Linux DEs and macOS offer this functionality out of the box. For Windows users there is WinCompose (free).

Basically, this adds an extra layer for the user allowing to use intuitive keys combinations instead of dull Unicode codes which are hard to remember. Pressing a Meta key initiates the input, and if it matches a program defined macros, a corresponding symbol will be inserted (Meta is typically the Alt key, by default right Alt aka AltGr); here are few examples which I personally cannot live without (I use US keyboard layout):

Meta o A = Å
Meta % o = ‰
Meta % % = ‱

Meta " a = ä
Meta s s = ß

Meta * l = λ
Meta * L = Λ

Meta / = = ≠
Meta + - = ±
Meta Meta o i i i n t = ∰

Meta o o = °
Meta E = = €
Meta o R = ®

Meta < - > = ↔
Meta </kbd> SPC = ␣

and so on. You can also tweak sequences in ~/.XCompose file and add your own ones; I only add a couple to struggle less when typing standard states and molecular geometry:

<Multi_key> <minus> <0> <minus>: "⦵"  U29B5  # PLIMSOLL
<Multi_key> <a> <n> <g> <l> <e>: "∠"  U2220  # ANGLE

This is by no means a complete substituent, but rather a nice tool which makes work with special characters less miserable and calling for a Character Map less frequent.