Deva, you really don't want to attach it in a way that restricts wood movement. Slotting or tenoning the verticals directly into the top, as you say, is the standard way, so theoretically when the top expands and contracts due to atmospheric changes, the verticals do at the same time. If you don't have the length needed in the verticals to slot them in, I can suggest another way. I think that drilling through the top for lags is kind of a shame, as you mention about least refined. Dowels will work, but I think a second best option to using tenons, in terms of strength. One other way if you don't have the length needed in the legs for tenons, is to glue and screw some cross strips in across the legs, say two or three per, wood or steel. Have them stick out wider than the legs.Then screw the legs to the top through the strips, with slots for the screws, so that the wood can move. Use lags and washers. You probably aren't set up for steel strips to drill, etc, but having thin pieces by way of steel, rather than the thicker wood that would be needed, makes for not seen. Just slot it (tenons) and drop it in using glue, the first method you mention, the most straightforward. A few tenons is better than one long strip. The tenons should be fairly long for that thick top, since there is not a stretcher between the legs. You might want to put one in, it will aid greatly to prevent racking.