Adding an SLMG to an NPPL(SSEA) involves "proof of conversion training" with an SLMG instructor.
Adding TMG privileges to an LAPL(A) involves a minimum 3 hours training, including at least 20 take-offs & landings, & then a skill test with an examiner. Good luck finding an EASA TMG examiner with access to a TMG.
You never lose an EASA LAPL, you just lose the privileges if you are out of recency. TMG hours maintain your recency for both TMG & SEP on an (A) licence but not for sailplanes on an (S) licence. However, it will be trivially easy to get back into sailplane recency if you ever want to in the future, so probably worth applying for it, & there's no downside to adding the TMG at the same time.