Why not increase the compression ratio? You can run 8.8:1 or even 9:1 on 98RON fuel without much trouble. Of course this requires proper mapping on a proper engine dyno by a proper specialist. Any attempt on a rolling dyno or on the road will end-up with 4 melted pistons.

Lowering the CR will push the knock limit somewhat further away but also RUIN the thermal efficiency and produce a sluggish engine with slow spool up and terrible response, which ultimately results in a slow car.

Consider that GrA engine (from 1996) were running 8.8:1 which was the maximum allowed by the regulation. Clubman WRC engine ran up to 9.6:1 and top international specs engines were above 11:1 before the water injection ban. OK, they ran on 102RON but 8.8-9.0 on 98RON is very doable and if you care about throttle response, spool up and lag, this is the way to go. Low CR is a relic from the days were ignition curves were rudimentary and difficult (if not impossible) to control precisely.

On the other hand raising the CR requires a good experienced specialist as you need to control the total chamber volume which include the piston bowl, valve pockets, gasket, head skimming etc. For 8.8 you want about 64cc total volume on std bore.

You must also pick some plugs (e.g. Denso IK31), measure the chamber volume with them, map the engine with them and stick to them forever: say your engine shows slight signs of det after hard driving on hot days, upon close examination of the plugs. Common (and wrong) belief recommends colder plugs = more ceramic = less chamber volume and 1cc difference makes your CR jump from 8.8 to 8.9 = more det... better add 5% toluene on track days.