Optical Digital audio out to AV cable

Mat you may know. The Hifi came with 5 speakers. I use the two big ones. It sounds Ok. I lost the centre one and two small ones. It has a pro Logic button. Is there any benefit in me adding a centre speaker with this method or will it just not work? I did add a post earlier. Hope you don't mind I have copied and pasted it below. Desperate for this info so much appreciated.

Ok. I have read through a few posts and can see the frustration sometimes where not enough information is provided to hopefully find a solution to a problem so I will try to be as comprehensive as possible but to the point.

My technical knowledge is limited.
I am not a perfectionist for picture and sound just want half decent quality.
I bought a new TV and the sound is poor. LG 4k led.
I am trying to find a solution to tie in with my other existing equipment but understand that I may need to spend more money and I am open to suggestions.
I have an old technics system. Technics SE-CH570
It has Dolby pro logic and a setting called Surround.
It came with Surround Speakers but do not have them any more but could but them.
It seems these would be connected via RCA connectors.

I figure I could use this to give me good tv sound although maybe not full surround sound.
I currently use this to run my Sonos via a Sonos Connect. It sounds pretty good. I am happy with this quality.

I think this is all of the detail.

The path I have gone down is this.

I bought optical cable and a DAC converter
It worked and sound is pretty good but I want to try to get a central speaker working for better quality.

So my questions is this. Is it worth me finding a set of the same surround speakers that came with the system originally. If I set the system to surround or Dolby Pro Logic will I get a surround sound or at least a centre speaker working adding to the quality. The original speakers are quite expensive and don't look that great. Can I just buy a centre speaker and then connect it to an RCA plug.

I understand that Pro Logic is very old not but I am not trying to achieve surround sound just decent sound from the tv through my existing amp if possible. By buying the original speakers again I am not even sure if they will work or if I buy a different central speaker if it will work.

Please if you feel I am going down a road which is just pointless feel free to give the bad news. I thought about buying a new amp but did not really want to spend £250 on the recommended merantz option advised to me.

Any advice here is greatly appreciated.

It may be worth noting I also want to add a record player to the system through the Phono in and play some old records I have. This worked through this stereo before no problem.

If anyone miraculously had some sort of solution where I could control the sound through my tv remote that would be incredible.