day trip along the Camino ending in Santiago de Compostela

What’s nice with the Camino is one can walk as much or little of it as one wants to. Not knowing what old equates to, you may wish to walk no more than 6 miles a day, or less/more.

Look at where you’ll already be on your trip. It’s possible one of the Caminos passes through it. If it does, start there. You could take a taxi 6 miles east then walk back to your hotel. The following day you could walk 6 miles west and take a cab back to your hotel. No changing of hotels would be required. My point is, there may not be a need to go through a company to accomplish what you want to do. If you want to visit Santiago you can take a bus or train to it.

The last stage of the Northern way is actually on the French way starting in O Pedrouzo, 12 miles from Santiago, and ending in Santiago. The last six miles is not very scenic with the last few miles being through the outskirts of Santiago to the cathedral.

It's worth knowing that to be awarded a Compostela you only have to have walked 75 km in Spain, if you have walked another 25km on certain specific pilgrim routes in the UK or Ireland, which were originally part of the Camino network, and had those miles recorded on an official Pilgrim's Passport.
From another post I note you are on Iona this September. One of the qualifying routes is the 62 mile St Andrew's Way from Edinburgh to St Andrews- so if you walk 25km (about 18 miles) of that Pilgrim route it counts towards the requisite miles in Spain.

In England one of the qualifying routes is the 68 mile St James' Way from Reading Abbey to Southampton- you only have to do 18 miles of that. Across England and Wales there are 6 other qualifying routes.

The other 75 km can be deemed to be on the Camino Ingles, starting from A Coruna (one of the historic arrival ports from England, along with Ferrol)- that route is also known as the Ruta del Faro-
https://www.pilgrim.es/en/english-way/stage-1b-a-coruna-bruma/
Vueling fly from A Coruna (and Santiago de Compostela) to Barcelona, Iberia fly from Madrid to A Coruna.
So maybe find a few miles starting out of A Coruna.

Let me clarify: We are over 80 and could walk up to 5 miles if it is relatively flat. I'd actually prefer a short tour bus trip for a day or 2 to drive us close to the route. Is that possible?

The Northern route merges with the French route for the last couple stages, so if you want to walk into Santiago to the cathedral it will be along the French way. There are companies offering day excursions that will drop hikers off for a few hours walk, then pick them up at a designated spot. Unfortunately, I don’t have any of their names.

For a 3.5 miles walk you could start at Homenaje al peregrino at Monte de Gozo. It’s just outside the town and you’d be walking through town to reach the cathedral.

The hotel in the town where you’d be picking up the tour should know which companies operate from it, so possibly send the hotel an email and ask. Or, the town tourist information office.

would like to at least glimpse part of the path and Santiago de Compostela

...

Let me clarify: We are over 80 and could walk up to 5 miles if it is relatively flat. I'd actually prefer a short tour bus trip for a day or 2 to drive us close to the route. Is that possible?

So, to clarify further, you want to visit the city of Santiago de Compostela AND want to see/walk a little bit of one of the Camino paths?

Somewhere like Lavacolla is 10km from the finish, so could find your way there and hike the final 6-ish miles from Lavacolla to the Santiago de Compostela Cathedral. Stay in Santiago de Compostela for a day or two (one for the Lavacolla to Santiago de Compostela hike) and then fly to Barcelona.

Separately, since the "Camino" is really a bunch of separate routes established over time, there are newer ones which pass near Barcelona (and Montserrat). Since you seem to want to visit Santiago de Compostela, that's less useful, but the various Camino options - as partial hiking paths - may be closer than you think in many parts of Europe - especially Spain, France, and Portugal.

Santiago de Compostela is one of my favorite towns. I recommend going.

If you head to the main square in front of the cathedral you can witness the range of emotions of the pilgrims finishing the journey: relief, exhaustion, joy, pain, determination, grief, disbelief. Then walking through the archway to the left facing the cathedral you can see where most of the pilgrims enter into the square and could follow that in reverse for a bit.

Personally, that would be a good way to connect with the spirit of the camino. And Santiago de Compostela is worth lingering a bit. It's a very atmospheric old town, the walk along the park looking across to the cathedral is nice, and there are a lot of good restaurants. Note that the overall town is hilly, but the old town itself is relatively flat.

I understand the desire to walk a part of the Camino, and the northern route looks very scenic, especially the area around Ribadeo. Unfortunately I don't have any direct experience with this and don't have any advice other than it looks like there are day trips available from Santiago de Compostela, and you may be able to find a Camino tour/logistics company that could provide transportation.

I find the walk from Lavacolla and on into Santiago to be very urban (walk past the airport and through the city) and rather uninspiring, other than the excitement of all the pilgrims. That is fun in itself. The walk from the Portuguese Camino into Santiago is nicer, but you don't come into the square through the passage where the bagpiper plays. Suggest you look at some videos to see if you want to walk into Santiago, or walk part of the Camino someplace else where it is beautiful (the path around Samos is lovely, as is the path around Morgade) and then bus into Santiago, hang out in the square.