We have an identically equipped Sedona in our stable. I successfully used Sram X-3 trigger shifters with the Shimano derailleurs. They cost me about $20 shipped (new) on Ebay. They are a well suited match for your 21 speed drivetrain.

For what it's worth: Our Sedona came with a Megarange freewheel, which is an actual freewheel and not a cassette. A cassette would be better, but the freewheel is okay on a Sedona. I switched mine out from the original 14-34 to a 13-28. This gave me even spacing between the gears, instead of having the large jump between the 24t and 34t cogs on the 14-34t. The freewheel is pretty easy to replace, using a Park FR-1 freewheel removal tool and a large wrench. The freewheel cost me about $20 at Amazon.com, and you can also purchase the tool from Amazon.

You may have to spend more money to convert to an 8 speed cassette than what the bike is worth to you, as you would have to switch out your rear hub or your entire rear wheel so that a cassette can be installed. An 8 speed cassette does have advantages over a 7 speed freewheel, but my opinion is that the advantages are probably not worth the cost.

Another problem you may have to deal with would be your bike's rear spacing is probably 130mm, as that is the usual spacing for a 7 speed freewheel, and uses mountain bike components. An 8 speed mountain bike cassette usually needs 135mm spacing. Your Sedona, unless it is a Sedona ST, has an aluminum frame that won't tolerate being respaced to fit a 135mm rear hub.

I would recommend changing out the shifters, but keeping the bike's 7 speed freewheel as is. If the freewheel is worn, I would recommend replacing it with a new 7 speed freewheel.