As for the bromination of tolualdehyde... Are the problems you mention with this reaction caused by the introduction of the formyl group deactivating the ring?
Exactly. It's the same thing with 4-methyl-acetophenone, it's very hard to brominate. You also need to use an extra equivalent of lewis acid to coordinate with the carbonyl oxygen when brominating - see http://www.orgsyn.org/orgsyn/chemname.asp?nameID=40843 (http://www.orgsyn.org/orgsyn/chemname.asp?nameID=40843)
Perhaps the direct bromination of p-xylene would proceed smoother, the bromine should still take the same position and the following oxidation should still proceed fine. just switch the steps.
I wonder if we could use the procedure they describe in Post 514596 (https://www.thevespiary.org/talk/index.php?topic=9411.msg51459600#msg51459600)
(Rhodium: "Many routes to 3,4,5-Trimethoxybenzaldehyde", Methods Discourse)
They react p-Hydroxytoluene with bromine to give 4-Hydroxy-3,5-dibromotoluene, which continues to become brominated on its methyl group, first forming 4-Hydroxy-3,5-dibromobenzyl bromide and next the benzal bromide (with a -CHBr2 side chain). This is then hydrolyzed with aqueous acid to give 4-Hydroxy-3,5-dibromobenzaldehyde in their example.
I wonder if the same procedure performed on p-Xylene would give 3-Bromo-Tolualdehyde, or if something weird happens?