We went through this last year. I would not try to cut a pre-fab vanity top down to size--the sides are already finished and they typically come with sinks so a lot of things to potentially go wrong there. Generally, you're just going to find sizes that correlate to stock vanity sizes, so 69" would be rare. Since you already have cabinets--and that may mean you already have a location for the sink that will not necessarily align to a stock top even if you cut it down--I would just go to a local shop and have these fabricated. You may be able to find a remnant, though we had no luck with that for quartz. It will be pricier than pre-fab but it will get you what you need.

More broadly, we bought our top from HD--not the original plan at all, but I couldn't find anything locally in our price range that worked for our space aesthetically. We ordered the top sight unseen figuring we could return it if it was really awful, and it shipped directly from the fabricator (who also sells direct to consumers). Overall I'm very happy with it--it arrived well packed, nothing was broken, and the quartz is nicer than I anticipated. We've only had it a year, so can't speak to whether it will hold up long term, but honestly for the price, it's okay if it doesn't. I was also pleasantly surprised by the sink it came with, which I assumed would be super cheap but looks good and so far works well. But we had a stock vanity size--and even at that, had slim pickings because it's a less common size (42") so just lucked out that there was something that would work. I don't love-love quartz overall, but that's about the product vs. the particular quartz, and we picked it because we didn't want the maintenance of marble in a bathroom, so I'm sure if we'd gone with marble I'd have different frustrations.

A few years ago I had several bathrooms renovated. I purchased remnant quartz countertops, for my new bathroom vanities, from a local stone yard (great cost savings too).

Peruse your local stone fabricator's remnant yard. Your color and bowl selection will be limited, but they are very interested in turning their mistakes into cash.