I actually have some Realistic hardware somewhere around, probably in the "mostly useless might be working old electronics" storage space out in the barn - near the defunct Standard Oil gas pump.
I think there's a stash of buggy whips out there too...
Using the headphones jack for amplified speakers is a Bad Idea (TM) - it auto-senses and adjusts other TV functionality based on it, it has variable (non-fixed) output, it has noisy output, and its analog.
If they really wanted people to connect amplified audio systems via the headphones jack, they should provide a mode setting (Fixed/Variable) for its output, maybe even "Analog/Digital" as well, for those circa late 90s/early 00s digital speaker systems....
TCL may officially support it, but its still a Bad Idea (TM), for all the reasons mentioned.
Of course, you are experiencing all of that first hand, hence why you are here now. Perhaps you found the amplified noise warm and soothing, reminding you of days of electronics glory past with a good old fashioned 60Hz hum and analog hiss...
Regardless, you could/should get one of the extractors/converters and hook it up either to the ARC or Optical port, and move the Chromecast to a non-ARC HDMI port (assuming one is available), for future "expansion" (read: Atmos AVR/Soundbar upgrade).
Most of the second-hand and smaller Mom & Pop electronics stores/outlets are gone - thankfully garage sales, eBay, and pawn shops exist to mostly fill that niche.
I always liked the real/faux wood-grained electronics casing myself even if it is dated, though you can have it (WARNING: The following is a UK Commonwealther import word used for purposes of mockery) bespoke made for current equipment by retro-enthusiast appealing entities willing to accept large donations from Boomer Boys...
Let us know how the audio extractor/converter works out, whichever one you decide to get.