Gummy loaf bread

May I ask you experts here why I almost always get a gummy crumb of my loaf bread? I can bear with the irregular holes since it is only for our family's consumption but the gummy crumb? I dont feel like Im doing a good job to keep my family off of grocery bread. I used to make a good loaf with good crumb and good crust using the recipes from KA Flours but I have a problem of the finished loaf being too heavy literally (not dense, just heavy). One finished loaf (using KAF paindemie recipe) baked in a 12.5x4in pain de mie pan would weigh almost 1.5 kilos! I wanted to find a recipe that will yield a lighter loaf so I kept on trying pain de mie recipes (P.Reinhart, C. Hitz, etc) but it almost always results in a gummy crumb. Or do my experiments mean I just need to stick to the KAF recipe? I need your help guys. I am desperate to get the perfect pain de mie recipe that could satisfy my family and me.

Other things I do are: 1) I knead by hand using slap&fold technique of R. Bertinet; 2) I do so for almost 30mins. I noticed by the 20th minute it still has poor gluten formation so I continue until 30mins; 3) Sometimes I forget to autolyse, but I do so when I remember 4) KAF recipe says to bake at 375F, 25mins with lid, 20mins without lid. But when I do it, it gets burnt (I have a brick oven). So I lower it to 300F instead and extend to 1 hour or 1hr 10mins max.

What could I be doing wrong?  ;(