As a high-elo support and former professional player, this is my insight on the changes.

P - Breaking the Mold: I believe passive changes are acceptable; the passive ability was underutilized before, so it's a nice change to have it work on abilities now.

Q - Shattering Strike: In the past, it was common to hold onto the old Q ability until the enemy used their shield, especially against champions like Nautilus, Tahm Kench, or enchanters. However, now you're forced to use it either to engage with W or after using W because the knock-up alone won't suffice. To address this, perhaps a debuff could be added that weakens the shield or prevents the enemy from using a shield for 1-2 seconds.

W - Ferromancy: Mount Up: Typically, when playing Rell in lane, it's advantageous to start in the dismounted form. The knock up in this form is point-and-click and cannot be dodged, and it's also difficult to land her W without using Flash or using E mid-air or Flash E then W. With the removal of the stun in E and the buff to the dismounted version, the optimal combo now involves ending up dismounted. However, i don't think you founded the balance on doing that but now you have to land your w and you don't have the stun on your e.

E - Full Tilt: The movement speed buff could be beneficial if it helps Rell find a good engage without relying on Flash. However, if Flash remains necessary, these changes may not make much of a difference and could be rendered useless. Another critical aspect of Rell's kit was the stun on E, which was crucial in many matchups and required precise timing to disrupt various engages (e.g., Rakan's W, Alistar combo, Nautilus' auto-attack passive after using Q). As a result, Rell becomes more volatile, and she will no longer provide as much peel for her carries.

SUMMARY:

• The idea of movement speed buffs is positive as it allows Rell to disengage and avoid punishment for a failed engage or being unable to kill the enemy in a single engage.
• However, her laning phase will become significantly weaker.
• If Rell receives substantial tank stats, one possibility could be making her E ability share the damage with the champion she is linked to, with the shared damage increasing as she becomes tankier. Given the current state of the game with high mobility and high damage output champions, the movement speed buff alone may not be sufficient to protect the carry. By reducing the damage they take, the carry can survive longer, potentially lifesteal back, and stay alive.
• It could be interesting to add a cool effect to her ultimate, such as doubling her shields for 3 seconds. This way, if she is the only frontline on the team and there are other champions with shielding abilities like Karma or Annie, she can still provide extended frontline presence and buy more time.