That’s great

. Drilling a tank is very nerve wracking, but it‘s good when it’s done. If you are able to take it outside and flush it out with a hose that might be best, but I can see how that might not work well in then winter. Second best would be taking it to the bath tub and flushing it out that way. (You might have to use little pitchers of water. Or if you can’t do that just use an old towel or some paper towels to try to clean it out as best you can. If a small amount of glass dust gets left in there I wouldn’t worry about it. It’s just going to get lost in the sand and it’s not going to hurt anything.
As far as the unions go, you could skip them, but skipping them could make life difficult if you need to do any work on the plumbing in the future. You might need to cut the plumbing to make changes. Having unions installed could make your life easier in the future. That said, I don’t have unions on my plumbing and I’m nearly 5 years and I haven’t regretted that decision too much yet.