Under current legislation, you need 'commercial level knowledge' even for PPL level instruction. Hence you would need to pass ALL the CPL exams, even if you didn't go on to complete a CPL Skill Test.
It is precisely for people in your circumstances that proposals have been drafted to change the regulations for PPL level instruction, with the knowledge that EASA will require that instructors must 'meet the theoretical knowledge and the experience requirements appropriate for the instruction being given' rather than needing 'commercial level knowledge' - but don't hold your horses as any changes would take several years to bring into law.
Many flying clubs would far sooner have the part-time services of an enthusiastic, experienced PPL-holding flying instructor than some wet-behind-the-ears hours-building wannabe 737 co-pilot. Not to tar them all with the same brush, of course, but some have little interest whatever in flying instruction and will be off to the airlines as soon as there's the slightest sniff of a co-pilot job.....