What about putting the single portal window after spring practice? Transfers can only matriculate in summer sessions. I don't know if it's feasible, but if transfers only have pre-season practice to acclimate to their new team, they may be less likely to jump ship. Keep the exception for the grad transfers as is though.
Here's the thing, I think the February-March practices are garbage.
The NFL gives time off after the season. Yes, that facilitates free agency and the draft, but it is also a time for players to heal.
For "spring practice" (Feb-March) you need multiple practices in shells and shorts before there is ever full contact in pads. So you already are not getting a full-go for a portion of the time. A lot of guys have not healed from the season, and now with the earlier portal window, you lose a lot of returning guys. And, sure, it might be fun to see the freshmen running around, but they've barely had a chance to get registered, let alone learn the playbook.
Start practice in late May/early June. Take a short break in July. Do NOT have an August mandatory "in shells" period, just pick up on August 1 where you left off on June 30.
Use the spring (for everyone, both returning players and freshmen) to learn the playbook, lift weights, etc. Maybe you allow for no-pads walk-throughs to get the timing and blocking of the playbook down.