Make Jello as firm as agar-agar
There is a modified form of tapioca that gives a clear solution, but it is not easily available and I do not think it would help in this case.

The obvious way to firm up Jello is to use less water than recommended - use somewhere between a half and three quarters of the amount specified on the packet.  But you are comparing agar with gelatin here, it is like comparing chalk and cheese. Gelatin is extracted from bones, agar-agar is extracted from seaweed.  They are chemically very different, the big common factor being that they form an edible jelly.

I'm not going to discourse on those differences, beyond saying that mixing chalk and cheese is not a good idea. I am just going to point out that, since much of the gelatin comes from pig bones, Muslims object to gelatin. OTOH Muslims like jelly, so in every Halal shop you will find a range of commercially produced fruit jellies which are mostly based on seaweed. Since vegetarians also object to gelatin, you get those jellies in health food shops and other vegetarian outlets as well.

Get some Halal jelly and start from that, you should be able to adjust the texture as desired with agar-agar ...