Our DM originally began by targeting two characters, the Sorlock and the Bladelock, by giving enemies Brooches of Shielding to nerf them. Realizing instead, he had just given us a new revenue stream, by killing these enemies and selling the brooches (naturally, after any party member who wanted one, got one). He did give one Boss a Mantle of Spell Resistance, now the Wizard wears it. He greatly regrets this choice. Our party averages around 9th level. Now the whining has changed to "most of the party are doing too much damage".

He has recently started giving all Bosses, and their assistants, Potions of Spell Resistance & Potions of Invulnerability that they drink immediately before the initiative roll. Now, everyone above a summoned peon, have both advantage on saves and halving all damage against them. Other than the Heightened Spell Metamagic option, which only the Sorlock has, is there any way to bypass this "nerfing" of our party?

It is difficult to counteract homebrew rules/items

"Spell resistance" is only ever specified under the 7th level class feature of Oath of Ancients paladin granting resistance to all spell damage to anyone in their aura. This homebrew is basically combining that mechanic with magical resistance or the use of a Rare magic item like the mantle of spell resistance, but doing so outside of that particular class. Coming up with a counter for a homebrew mechanic that provides advantage on all saves AND resistance to all damage types from spells becomes difficult because the RAW mechanics don't really account for this.

There are mechanics like Heightened which force disadvantage on a save and the feat Elemental Adept that lets you ignore resistance for a specific damage element - but a total mechanic of resistance to all damage from spells or ignoring advantage on saves doesn't exist because that isn't a RAW ability in general.

You need to talk with your DM

This sounds more like an issue at the table than an issue with mechanics or using mechanics to counteract in-game difficulties. Talk to your DM about why they feel the need to do this. They're basically granting Magic Resistance to all monsters (and CR should be adjusted accordingly) as well as giving them each the potion of invulnerability, a rare item that only lasts a minute (but maybe extending the minute and giving them a 'free' action to drink it.)

Either way, there's a deeper issue that needs to be resolved. Working around it will unlikely be successful.

Since my witty comment has been unceremoniously deleted...

5E doesn't have much in the way of bypassing spell resistance.

To answer your question:

Weapons - Their damage isn't affected by spell resistance. Have at it!

To address the underlying problem:

Your GM isn't trying to nerf your party as you suspect. Their job is to run a difficult or challenging encounter for your party while making everyone in it feel relevant (or give them a turn to feel relevant). This includes letting the casters who rely on their spell #1,2 and 3 to sometimes think outside their box.

There are different methods of achieving this. Each party is different, therefore each GM will follow different methods. Don't expect your GM not to adapt encounters to counter your party's strengths instead of simply playing on it's weaknesses.

If each fight ends quickly with big spells your encounters will become dull, boring and repetitive. To me, D&D wasn't designed to be a grindfest like MMORPGs, but rather a narrative driven story of overcoming struggles with heroic acts and feats of strength, intelligence and wit.

Your role as a player is to overcome the challenges imposed by the GM. Sometimes they are trivial, sometimes they are challenging. The aim is that they are always fun, otherwise why even bother?

Don't endeavor to 'outplay' your GM. D&D isn't a 'you vs. them' game. They aren't 'against' you, but rather 'for you to have a great time'.

If you feel that your GM has a vendetta against you, I recommend you bring it up with them. If you want to 'facestomp' baddies more often, tell your GM. As long as its fun and falls within his plan for your scenario I'm certain he would indulge you.