I definitely think I react less than I used to. When I first started hillwalking, I'd only get to midge country a few times a year, and the rest of the year I hardly got bitten by anything. I reacted badly, especially to midges, and it was quite uncomfortable and lasted a few days for each bite. I don't know if this is related, but these days, I spend time almost every day outdoors, and for much of the year, I get bitten by something most days. It does seem to help build up a resistance and I don't react much now. Have to say, I'm not sure cleg bites specifically have been better than before, but midge and mozzie do seem to be.
On the most recent trip, I also took a "one a day" antihistamine tablet.
I use Smidge. It definitely reduces biting, but doesn't stop them landing and they seem to be able to find the bit you missed more than with DEET, which I too no longer want to use. It needs applying more often than strong DEET used to, but doesn't dissolve your carrier bag print, watch strap or handlebars as DEET has done!
Some clegs are equipped with laser drills or something and can eat you through clothing!