The series 4 line of TCL's are meant to compete with RCA/Hisense/Sharp budget models along with Vizio's M/V series, the 6 series are in a completely different level with brightness peaking over 1000cd/m², most of those others peak out at 300-400 which doesn't give a very dynamic visual, especially in 4k HDR10.
One other BIG thing to consider is does the tv have a processor to Upscale an SD/1080p image to 4K, the pricier sets do and it DOES make a big difference, some upscaling engines are better than others, rtings.com offer some great comparisons.
Sharp licensed their TV line to Hisense and have since regretted it and are trying to get it back, poor quality, and poor specs were the reasons.
As I said before I did a LOT of due diligence when looking for a new tv and my only concern with the TCL brand was hundreds of negative reviews for Dirty Screen Effect, if you watch sports or play games this is the one thing that can really ruin the experience.
, you can buy an SVHS to HDMI converter for less than $30 on amazon. Also, if you're unhappy with the tv remote you can download a free app off Google play that offers a more traditional remote, just use your tablet, laptop or phone and you're good to go.