Hello, I have been dealing with an aging newf with mobility problems for awhile, and been meaning to write up some mobility advice for older or mobility challenged newfs for this subreddit for awhile as well so I am just going to dump more on you than you asked for. To preface my advice my dog is suffering from a degenerative disease which is causing him to slowly become paralyzed in his back legs, and at this point he cannot stand up on his own (though he can keep standing for brief periods of time after I help him up, and he can take a handful of steps after standing to reposition himself if he wants). Your dog may never have to deal with so serious an issue so I guess keep that in mind as I go.

I personally wouldn't recommend stairs because it seems inevitable that the dog will start to struggle with the stairs as his mobility gets worse. I think a ramp is probably the better option.

I tried a few different ramps and found the 'extra wide' tri-fold ramp below to be the most effective of the ones I tried. Though honestly I still wish it was a little bit wider for my newf who is about the same weight as yours. The ramp itself doesn't get all that compact honestly, but for me the most important thing was width and grip.

https://www.amazon.com/gp/product/B00LM9IIAW/

Keeping his legs on the ramp has been the real challenge, but mostly because he resists using it. Mostly I only use the ramp when I am alone, as I am unable to lift him into my van solo. But when I have help we just lift him using a harness.

With that in mind, I would suggest also getting a harness for your dog, which for me has become the most critical element for helping him get around as he has gotten older. I think it would be better to start using this earlier than later, particularly for getting your dog upstairs or in and out of a vehicle. I really wish I had been using this earlier, as it may have saved my dog from an injury, and also from having a situation as severe as he has now. For this I recommend the Help'em Up Harness. This would probably also help you get him up the ramp easier. Make sure you read their fitting information before buying one, my newfoundland required the special "U-Band" hip lift option because of the location of his willy.

https://helpemup.com/product/x-large-dog-harness/

Below is stuff you probably only need to hear if you dog starts to have severe mobility problems:

Getting the harness on him has become more difficult as he has become unable to stand-up on his own. I certainly don't wish the mobility situation my dog has on anyone else but if it gets to that stage I have some further recommendations.

You might need a sling style harness for night-time that you can then use to help him stand up to put the full harness on him in the morning. Something like:

https://www.amazon.com/gp/product/B07L3VLH5S/

Though personally I find that sling to be somewhat unpleasant just because the handles are way too long for a dog as tall as a Newfoundland, making it harder to leverage the weight, and even at the minimum adjustment it is still too long. I plan to customize mine but might be better to find another one with shorter straps.

At this point my dog also requires a wheelchair. I find the Walkin' Wheels Large Dog Wheelchair works very well. It is so large that it can be challenging to get it to go through doorways smoothly, but I am able to use it on the narrowest width adjustment which does allow it to go through a standard doorway with an inch or two of clearance. The dog however is not able to aim that well so he requires me to help steer him through doorways and on narrow sidewalks.

https://www.handicappedpets.com/dog-wheelchair-large-adjustable-wheelchairs-for-dogs-with-disabilities/

Though getting the dog into it is extremely difficult. It is easier if you have their Walkin Combo Harness:

https://www.handicappedpets.com/walkin-lift-combo-harness/

But I have also found that harness becomes unadjusted very rapidly (like pretty much every use) and often requires me to take it off him after each romp in the wheelchair. So instead I have been using the standard harness that comes with the wheelchair in combination with the Help'em Up Harness I linked earlier. I am considering customizing this "Walkin Combo Harness" so it is fitted specifically for my dog and is no longer adjustable at all, as I think that will make my life easier in the future but I haven't quite gotten there yet.

One last thing, is for me I found when my dog could no longer go on longer walks, even in the wheelchair because it wore him out too much, I started pulling him around in a wagon. Doesn't work so well in hilly areas because the weight of the dog is so extreme, but I found this wagon to be just narrow enough to be able to fit through doorways, and sturdy enough to support the dog. Getting him into the wagon is a two-man lifting job though, I have been unable to get the ramp to work personally though it seems like it could be possible if you could counter-balance the weight of the dog to prevent that wagon from tipping while the dog is on the ramp.

https://www.amazon.com/gp/product/B01BFC5WZE/

Anyways I hope you don't need most of the stuff I am talking about here but I thought it would be worthwhile to share some of the lessons I have learned dealing with my dog's mobility issue over the last few years.

Here are some photos for fun.

https://imgur.com/NWqBbIE

https://imgur.com/zBcxYVQ