You can't use NaOMe to do a williamson ether synthesis on an aromatic halide, this sort of chemistry can only be done with alkyl halides. To make your 2,4,5- trimethoxy you're going to have to form the phenol from the bromide made with NBS, deprotonate it, then methylate with dimethyl sulfate, methyl iodide, etc.
Read https://www.thevespiary.org/rhodium/Rhodium/chemistry/345-tmb.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/345-tmb.html)
for the general idea.
Look here for a possible improved procedure
Post 287645 (https://www.thevespiary.org/talk/index.php?topic=7652.msg28764500#msg28764500)
(foxy2: "3,4-Dihydroxy-5-methoxybenzaldehyde", Chemistry Discourse)
Those who give up essential liberties for temporary safety deserve neither liberty nor safety
Chromic: Copper catalyzed substitution of aromatic halides with methoxide are indeed possible.
See this reference for example: https://www.thevespiary.org/rhodium/Rhodium/pdf/arylhalide.methanolysis.cu-etoac.pdf (https://www.thevespiary.org/rhodium/Rhodium/pdf/arylhalide.methanolysis.cu-etoac.pdf)
Chromic (and Rhodium)
nothing against the Chief bee's recommendations, but Uemura may recommend also to you to read the 'Vanillin' thread (see Post 274828 (https://www.thevespiary.org/talk/index.php?topic=6626.msg27482800#msg27482800)
(Natrix: "Vanillin", Chemistry Discourse)). Perhaps it helps to avoid some frustration. :)
BTW: are there any succesful reports on copper based methoxylation of arylhalides without DMF around in the Hive? Uemura would bee realy interested in!
Carpe Diem