I am a Gabbi of last resort at my weekday minyan and although we have a light small sefer torah many people refuse hagbaha. I have spoken to some sphardim ( those from middle-eastern descent) and they too complain about heavy sefer torahs, in their case it the case holding the sefer which is covered in gold and silver making it hard to carry.
The way most people do hagbaha on a heavy sefer is to pull the bottom of the sefer so it about a quarter off the table then bend your knees and press down on the etz chaims. You will be using leverage to lift the sefer up. The only problem with this method is that it puts pressure on the klaf and if the ink is old and dried out it can crack over time. This is the problem with the old sefer torahs. Once it starts cracking in one place and you repair it a little later it happen somewhere else. Pretty soon it’s going to cost you more to repair it then to buy a new one.
I have no idea of what your shule Aron looks like, but if you have 8 sefer that are not being used, Are the families active in your shul? Many families may have moved away or there financial situation may not allow them to maintain the their sefer. Sooner or later you are going to run out of space in the Aron if new Sefer Torah are donated.
The solution maybe to repair these old sefer torahs and replacing the etz chaims and montale, (covering) and rededicate them. It would probably be cheaper than having a new sefer written and the extra money can be used for it’s upkeep. Check with the rabbi and the owner of the sefer before you try to do this.