Unknown problem after some work i’ve done, please help

Check youtube video first:

There were no major problems with the car, except for one issue: when sitting in traffic, if the battery discharged and the gauge dropped to two bars, a slight vibration would appear when the gasoline engine kicked in. During the time it took for the battery level to rise from 2 to 3 bars, the engine didn't run completely smoothly (but nowhere near as rough as in the video).

P.S. This issue was present both before and after the LPG system installation.

I've also noticed that when pressing the gas pedal harder uphill, there's a slight pulsing sound (not reflected in the RPMs, which rise smoothly), so I didn't pay much attention to it.

Recently, there have been two instances while driving where the car suddenly lost power, as if the gasoline engine had shut off. I continued driving on the battery until it fully discharged, and the car came to a complete stop. After turning the car off and restarting it, everything seemed fine. Diagnostics showed two error codes: Poor Engine Power and Engine Failed to Start. After clearing the codes, the car drove normally again.

Today, trying to address these potential issues as a preventive measure, I:
- Replaced the EGR cooler and EGR valve (both were cleaned and shiny).
- Installed new spark plugs (one of the four plugs was hard to unscrew and had some slight corrosion).
- Changed the transmission fluid.
- Reassembled everything, but now it's clear something is wrong when starting the engine.

I kindly ask for your advice on where to start looking for the problem. What might I have done incorrectly, and where should I check?

No error codes are showing on startup.

Car details:
- Toyota Prius 2009 XW30, 2ZR-FXE 1.8, gasoline-LPG
- Spark plugs used: Denso Iridium IXEH20TT
- Transmission fluid: ATF T-WS Ravenol