You could try to control the water flow by pinching the hose a bit. Some siphon sets even come with some plastic xontraption to do just that. Although slowing the flow takes from the vacuum action, it also helps with not taking too much water too quickly. Maybe you could also try to rig a pvc pipe with a smaller diameter so that you don't lose vacuum power but reduce the volume of water that it moves.So I actually had a question about gravel cleaning cause I just did one today. I think my vacuum is to big, I think it was made for much larger tanks. It worked but my vacuum is too wide and long. I was only able to clean maybe half my tank and it took up about 50% of the water. Which was okay because I needed to do about a 50% water change( because of high Nitrates due to plant fertilizer, I usually do about 10% once a week and that keeps my parameters in check). With my little 10g tank I think I need a smaller vacuum, I was hoping to vacuum whole tank. When I transfer over to 20gal long, I'm switching to sand substrate for my Cory Cats. ( Before you criticize, I already know, I didn't know about sand and Cory's when I got them, that's one of the reasons I'm upgrading, so please don't hate me in the comments, I'm working to correct it, and my Cory's are doing okay and are healthy at the moment)
Congrats on changing to sand for cories, I went thru the exact same thing when I first started my tank back in December last year.
Yeah, a bigger tank will help with the fish, 3 more cories would make them a happy bunch, and the female bettas usually work better in community tanks than a male would.
I'd go for a 30g or even 40g with the plans you have in mind, don't short yourself now and go thru another tank change a few months down the road.