Pup refuses to go down steep stairs and getting too heavy to carry

Hi! I’m in a bit of a pickle and looking for advice.

My pup is 18 weeks old and about 20 kg / 44 lbs. Because of his age and joints I’ve been carrying him down the stairs to go potty, but he recently developed a new… quirk. He now signals that he needs to go outside by sitting at the top of the stairs, but the moment I try to pick him up he runs away from me in circles, hides under the table or flops down and fights me. Most of the time he’ll end up peeing or pooping under the table while I’m trying to get him back to the stairs.

Even if we overcome his dislike of being picked up, I won’t be able to keep doing it much longer so I really need to teach him to go down the stairs on his own.

The problem is that this staircase is steep and narrow. He has no issue going up and down “normal” stairs elsewhere, but he’s just too afraid of going down this one. We've been working on him going down the stairs for a couple of weeks now. I’ve tried using high value treats to encourage him, helping him by supporting his body, manually moving his paws down and starting from the bottom step by step. He will go down the second step from the bottom, but once we get to the third step he freezes completely and won’t move.

I’m not sure how to progress from here and I’m worried that this behaviour is turning into a BAD habit. Plus, physically I won’t be able to keep carrying him for much longer (I'm 5'3/160cm 105lbs/47kg) as he will soon be heavier than me.

Any ideas on how to help a pup gain confidence with a steep staircase or am I destined to start weightlifting? lol