Using Methyl iodide would probably make more of the quaternary ammonium compound than 5-MeO-DMT.
I suggest you use the procedure in the document below (for synthesis of DMT) and just substitute your mexamine for the tryptamine:
https://www.thevespiary.org/rhodium/Rhodium/chemistry/tryptamine2dmt.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/tryptamine2dmt.html)
NaBH4 is untested. It could work with the use of a drying agent, like in the reductive amination of MDP2P with methylamine, but noone knows that yet. A possible problem is the reduction of the pyrrole double bond by formed triacetoxyborohydride in the presence of acetic acid. Maybe by omitting the acetic acid completely, it would work just fine, see Post 243844 (https://www.thevespiary.org/talk/index.php?topic=12768.msg24384400#msg24384400)
(Rhodium: "Tryptamine reductive alkylation with NaBH(OAc)3", Tryptamine Chemistry).
5-Cyano-DMT from 5-Cyano-Tryptamine
0.121g (0.545 mmol) 5-Cyanotryptamine HCl was dissolved in 25ml methanol, and with good stirring 0.1ml 30% methanolic sodium methoxide (1 eq) was added, followed by 0.125ml HOAc (2.18 mmol, 4 eq) and 68mg (1.09 mmol, 2eq) NaBH3CN. Next 0.103ml 38% aqueous formaldehyde (1.36 mmol, 2.5 eq) in 10ml methanol was added over 30 minutes. The solution was allowed to stir for 3.5h at room temperature, and 25ml 4M NaOH was added and the methanol evaporated off under vacuum. The residue was extracted with 3x25ml Et2O, the organic extracts washed with 2x20ml brine, dried over MgSO4 and the solvent evaporated to give 5-Cyano-DMT in 75% yield.
Ref: J. Med. Chem. 44, 3881-3895 (2001) (https://www.thevespiary.org/rhodium/Rhodium/pdf/reductive.tryptamination-1.pdf)
(https://www.thevespiary.org/rhodium/Rhodium/pdf/reductive.tryptamination-1.pdf)
This method should be general for all tryptamines. The same article also describes the reaction of tryptophol with phenylsulfonylchloride to give the N,O-diphenylsulfonyl derivative in 50% yield, which is then reacted with a dialkylamine and K2CO3 in IPA to give the 1-phenylsulfonyl-N,N-dialkyltryptamine in 65% yield. Is there any way to deprotect that derivative in good yield?