I assume OP is pretty dedicated to using a backpack.
Mine is an ugly $4 thrift store find, but it was unused. It fits my 15.6” work laptop and a change of clothes. Or about 2 bags of groceries. In addition to my bike tools, spare tube, mini pump, and lock if I’m carrying one. Wallet, keys, all the personal effects.
I started using a backpack years before I started commuting by bike. They’re just practical.
A cheap rack and pannier that my backpack fits into works pretty well for me. Keeps my backpack clean and dry and off my back while I’m on the bike. I get the added bonus of more than double the hauling capacity for grocery store trips, I can wear the backpack AND load the pannier AND strap a bag or two to the top of the rack. Or I can take the pannier off completely, strap the pump, patch kit, and tools and maybe some food to the top of the rack for a day trip. Rear racks are generally inexpensive and will fit almost any bike. Panniers can be pricey but people do sell them used. To go back to carrying all that weight on my back and dealing with the additional sweat at this point would feel like punishment.
Edit: Additional thought - Handling is very different for me, too, whether load is on my back or on the bike, but I don’t generally commute aggressively, I tried for a few months and was never able to realize the benefit. It was just more sweat and more pain, I never really saved any time. Maybe I just wasn’t aggressive enough. But I would not describe the difference in handling as positive or negative if not riding aggressively, just different. If I were to want to ride aggressively, I wouldn’t want any added weight on my body or the bike honestly.