[Quoth]

Originally Posted by xavier1

I am wondering if this pen (or stylus) has some lag when using it, or if the drawing experience is smooth?

That more depends on the host system. There is lag with the eink reMarkable and none with a Lenovo X201T with Wacom. The lag is different on the Kobo Elipsa and Sage and the same pen has none on an MS Surface. I've used two 3rd party NTrig/MPP pens for Surface 3 or later (without the BT option) and the Kobo pen. The pen makes no difference, but what you use it on does.

The TCL T-Pen is perfect on the NxtPaper 11. Better than Wacom on laptop or Apple pencil on iPad. Likely any USI 2.0 pen is the same.

Originally Posted by xavier1

Is this pen universal, so I can use it with the TCL or other tablets?

Only other tablets that use USI 2.0

There are some 3rd party pens that are multi-protocol. Most are not.
In terms of active pens (some, including all Wacom models, are remote powered), the following are incompatible. However some tablets may support more than one protocol:

1. MS NTrig/MPP used on Surface 3 and later, also Kobo. Some versions have an extra Bluetooth button used by OneNote. Battery and rechargeable versions. Third party versions.
2. Apple Pencil. Rechargeable only. Third party versions.
3. Wacom (earlier models). Remote powered. Various button options. Older laptops and digitizer pads such as Bamboo.
4. Wacom EMR. Though all Wacom pens are really EMR based. The reMarkable and Kindle Scribe. Third party versions exist.
   
5. There may be a 3rd Wacom version. Not sure which Wacom is used on MS Surface 1 & 2.
   
6. USI 2.0. The TCL T-Pen and others. Battery and rechargeable versions.
   
7. I don't know which system the Samsung S-Pen uses.

There are also some obsolete styluses with BT and pressure sensing for tablets without a digitizer. Only specific apps use them. Capacitive touch existed since 1980s but was rarely used as resolution for Windows like GUIs, "graffiti", annotation, signatures etc was wanted, so high resolution resistive touch was used up to 2010. IR touch is even lower resolution and was used from 1980s on CRTS for interactive systems on shops and museums. It was used on earlier eInk because capacitive and resistive touch need transparent metallic layers in front of the panel which reduce brightness and contrast. A digitiser is a layer behind the display with the pen having a pressure sensor. The system can be the pen sensing location and transmitting it, or the digitiser sensing the pen.

Originally Posted by xavier1

What do you think about the idea of making tips with 1.2 mm trimmer line? Would they fit inside the original T-Pen?

I'd not bother. The bought tips have a nice rounded point and only the lightest touch, like a liquid ink pen (nib or capillary or brush tip), is needed. A Pentel roller ball needs more pressure and a regular pencil or ballpoint needs much more pressure.

The tip has to fit well so it's held by connection to pressure sensor and yet moves smoothly without friction in the opening of the pen. Some pens use a different tip scheme.

The tips should last a long time unless abused.