Yes, you’re absolutely correct, the downstream SNRM is way way too high. This is a bad thing and a good thing. It’s a bad thing because it slows you down. It’s a good thing provided that it is necessary and not just a screw-up within the BT Dynamic Line Management system which responds to detected fault symptoms by raising the target SNRM and taking other steps to make error correction far more powerful. If a high downstream SNRM is unnecessary then the target SNRM needs to be fixed, by a call out from an Openreach engineer, iirc. That would be arranged by your ISP. If a high downstream SNRM is necessary, then it is doing a good job for you and protecting you from errors caused by you line being bad; most likely either the line is faulty or it is picking up interference. That would be up to your ISP to diagnose and fix with an engineer callout. If the ISP can’t understand this, can’t diagnose it or manage a callout effectively then you need an ISPectomy, removal of the current ISP and replacement with a competent one. Either way, it’s over to your ISP next and see if they even know what a ‘downstream SNRM’ is. If they tell you to ‘turn it off and on again’ then I think you already have your answer.
You asked about different modems. The HG612 is a good but now very old modem and their components are deteriorating, as with pretty much all such kit. My own ZyXEL modems are pretty old too so I can’t talk. Unless it has developed a fault, which is I think unlikely to be the cause of any problem now, the HG612 will be fine. Were I in your position I would change it now because you will have to one day, before it dies anyway, and I would swap it out for a top of the range ZyXEL modem such as a VMG 8x24-B10A because they are superb and also have best-in-class detailed stats which will make it possible for us to help you to diagnose what’s going on now, and will allow you to keep an eye on future behaviour. Kitz, Burakkucat and I myself are amongst the many members who all swear by ZyXEL modems. The diagnostics available to you and which you posted above aren’t sufficiently detailed for us to make a full assessment.
Be aware that some ISPs are very unsupportive if a customer asks for help when using a modem or router that the ISP didn’t supply. This is not totally unreasonable but it’s extremely unhelpful to the customer. If this should happen, then it’s another reason to recommend an ISP switch.
Keep in touch, come back with further questions and do let us know how you’re getting on.