I’ve looked at your location and according the footprint maps a 90cm dish is sufficient so I doubt you need to change the 100cm dish. It should be more than adequate. You write that you have checked the cables. I always cut them back at the LNB end by 10 or 20 cms and re-make the connectors. Water can get in and cause them to deteriorate. Then look at the LNB, a physical examination to ensure it is not cracked.
2/3rds bar should produce a sufficient signal for HD. I have an octal LNBs with a twin cable to my Humax Box and they both produce different signal levels, one of which is barel 2.3rds.
Before re-aligning the dish I always swap the LNB to see if that is the cause because simply the last thing I want to do is climb a ladder and mess with the dish.
Finally with a good meter test the alignment and adjust as necessary.