eTeX instead of TeX => ! You have to increase POOLSIZE.

[CarpeDiemKopi]

No problems if I take exactly your instruction steps:

1. tangle -underscore tex.web
2. ...
3. compile.js
4. initex.js

If I try with eTeX:

1. tangle -underscore tex.web etex.ch
2. renaming etex.src to plain.tex so the format file fits
3. ...
4. compile.js
5. initex.js
   => error:
   ! You have to increase POOLSIZE.

I have a TeX Live 2019 distribution under Windows 10, for more background of this issue see TeX SX: Detailed description of my case

Your example drawing a circle with TikZ works with eTeX:

This is e-TeX, Version 3.14159265-2.6 (preloaded format=latex 2019.2.23)
**entering extended mode
(sample.tex
LaTeX2e <2018-04-01> patch level 5
(sample.aux) ABD: EveryShipout initializing macros [1] (sample.aux) )
Output written on sample.dvi (1 page, 2976 bytes).
Transcript written on sample.log.

How to fix this?
What am I doing wrong?

I think my etex.ch file doesn't contain all necessary changes, e.g. memory managament pool_size ....

Do you have a working etex.ch or ready-to-use etex.web?

[CarpeDiemKopi]

Marcel Krüger provided a solution see: TeX SX answer.

[CarpeDiemKopi]

I have still memory management problems of other than only plain formats, see:
TeX initex error “TeX capacity exceeded, sorry [pool size=8563]” while constant pool_size is set to 350000. What can I do?

[jhoobergs]

@CarpeDiemKopi Maybe #5 fixes you problems ?