Downgrading to 2020.3, since none of the service package dependencies need a higher version. I've also removed a few unused packages, mostly for 2D sprites which we don't need.

Packages/manifest.json

 {
   "dependencies": {
-    "com.unity.2d.animation": "6.0.3",
-    "com.unity.2d.pixel-perfect": "5.0.0",
-    "com.unity.2d.psdimporter": "6.0.0-pre.2",
-    "com.unity.2d.sprite": "1.0.0",
-    "com.unity.2d.spriteshape": "7.0.0-pre.2",
-    "com.unity.2d.tilemap": "1.0.0",
     "com.unity.collab-proxy": "1.5.7",
     "com.unity.ide.rider": "3.0.7",
     "com.unity.ide.visualstudio": "2.0.9",
     "com.unity.test-framework": "1.1.26",
     "com.unity.textmeshpro": "3.0.6",
-    "com.unity.timeline": "1.6.0-pre.5",
     "com.unity.toolchain.win-x86_64-linux-x86_64": "0.1.20-preview",
     "com.unity.ugui": "1.0.0",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.vr": "1.0.0",
     "com.unity.modules.wind": "1.0.0",
     "com.unity.modules.xr": "1.0.0"
-  } 
+  }
 }

Packages/packages-lock.json

 {
   "dependencies": {
-    "com.unity.2d.animation": {
-      "version": "6.0.3",
-      "depth": 0,
-      "source": "registry",
-      "dependencies": {
-        "com.unity.2d.common": "5.0.0",
-        "com.unity.mathematics": "1.1.0",
-        "com.unity.2d.sprite": "1.0.0",
-        "com.unity.modules.animation": "1.0.0",
-        "com.unity.modules.uielements": "1.0.0"
-      },
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.common": {
-      "version": "6.0.0-pre.2",
-      "depth": 1,
-      "source": "registry",
-      "dependencies": {
-        "com.unity.2d.sprite": "1.0.0",
-        "com.unity.mathematics": "1.1.0",
-        "com.unity.modules.uielements": "1.0.0",
-        "com.unity.burst": "1.5.1"
-      },
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.path": {
-      "version": "5.0.0",
-      "depth": 1,
-      "source": "registry",
-      "dependencies": {},
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.pixel-perfect": {
-      "version": "5.0.0",
-      "depth": 0,
-      "source": "registry",
-      "dependencies": {},
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.psdimporter": {
-      "version": "6.0.0-pre.2",
-      "depth": 0,
-      "source": "registry",
-      "dependencies": {
-        "com.unity.2d.animation": "6.0.1",
-        "com.unity.2d.common": "6.0.0-pre.2",
-        "com.unity.2d.sprite": "1.0.0"
-      },
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.sprite": {
-      "version": "1.0.0",
-      "depth": 0,
-      "source": "builtin",
-      "dependencies": {}
-    },
-    "com.unity.2d.spriteshape": {
-      "version": "7.0.0-pre.2",
-      "depth": 0,
-      "source": "registry",
-      "dependencies": {
-        "com.unity.mathematics": "1.1.0",
-        "com.unity.2d.common": "6.0.0-pre.2",
-        "com.unity.2d.path": "5.0.0",
-        "com.unity.modules.physics2d": "1.0.0"
-      },
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.2d.tilemap": {
-      "version": "1.0.0",
-      "depth": 0,
-      "source": "builtin",
-      "dependencies": {}
-    },
-      "version": "1.6.0-pre.2",
-      "depth": 1,
+      "version": "1.5.3",
+      "depth": 2,
       "source": "registry",
       "dependencies": {
         "com.unity.mathematics": "1.2.1"
       "source": "registry",
       "dependencies": {
         "com.unity.ugui": "1.0.0"
-      },
-      "url": "https://packages.unity.com"
-    },
-    "com.unity.timeline": {
-      "version": "1.6.0-pre.5",
-      "depth": 0,
-      "source": "registry",
-      "dependencies": {
-        "com.unity.modules.director": "1.0.0",
-        "com.unity.modules.animation": "1.0.0",
-        "com.unity.modules.audio": "1.0.0",
-        "com.unity.modules.particlesystem": "1.0.0"
       },
       "url": "https://packages.unity.com"
     },

ProjectSettings/ProjectVersion.txt

-m_EditorVersion: 2021.2.0b1
-m_EditorVersionWithRevision: 2021.2.0b1 (b0978dae4864)
+m_EditorVersion: 2020.3.14f1
+m_EditorVersionWithRevision: 2020.3.14f1 (d0d1bb862f9d)