How to get a serial reading from an Ultrasonic Rangefinder on a Raspberry 3 B+?
First of all, sensor TX pin is transmit output so it must be connected to RasPi RX pin which is a receive input - Connecting TX to TX is usually wrong. But in this case is not that simple, so don't do that just yet.

The sensor TX output voltage levels are same as it's supply voltage, so it uses 5V logic levels. RasPi RX pin does not tolerate 5V logic levels, it must use 3.3V logic levels, so some conversion is needed. Fortunately the output might be so weak that the RasPi RX pin can tolerate it, but this is definitely not good to drive 5V signals to things expecting 3.3V signals.

Finally, the sensor TX output is actually directly compatible with RS232 voltage level receivers, which means it must be inverted before connecting it to a UART without RS232 voltage receiver such as RasPi. This is the reason you don't receive correct characters.

Solutions are to connect a USB-RS232 adapter to RasPi, or add a voltage-level converting inverter in between. For example a 74LVC04 is an inverter, it can be used with 3.3V supply voltage to drive RasPi RX input with 3.3V logic level, and it can tolerate 5V logic levels on inputs.

However, those are not the only solutions. The sensor can be used with 3.3V supply voltage, so you can use the sensor and inverter both at 3.3V, so any inverter that runs with 3.3V can be used, such as 74HC04. You could also get a RS232 to 3.3V TTL/CMOS level receiver board that runs at 3.3V to do the inversion and level conversion.