The Problem
By changing the user controlled time to "start of burn", the KSP2 maneuver planner will never be as useful as the KSP1 planner. I am assuming that a fix will soon allow us to expand and lock the details in the Pe/Ap and other points. Even when that works again, the new maneuver planner will still be worse than KSP1. By increasing the delta-v with the start time fixed, you're actually changing two variables: the amount of delta-v applied and the effective time it's applied. You'll have to make at least one other compensating adjustment to correct this issue and it will always be a worse experience.
KSP1's maneuver planner was a fine educational tool (change the input, see the effect on the output) but you could never graduate from playing with the inputs to specifying the outcome. There are a couple of real improvements that can be made to the KSP1 planner without losing the game vibe, and two steps beyond, for those who want to leave the wacky variability of manually executing inaccurate maneuvers to a time when they were getting the hang of things.
A Possible Solution (that stays true to the game)
I suggest there should be two planners and an autopilot that players can opt into via the tech tree. The two planners are 1) a slightly improved KSP1 planner and 2) a "Kerbonautic" maneuver planner. The autopilot can put you on the maneuver vector, start and stop the burn precisely.
Slightly Improved KSP1 Planner
The two improvements I would make to the KSP1 planner are 1) the maneuver vector should be relative to prograde and should follow prograde during the burn and 2) you should be able to set the engine throttle for a maneuver.
On the maneuver vector:
Orbit angle adjustment maneuvers are more efficient if you burn orthogonal to the path (lock to the normal or anti-normal) and not on a fixed vector. For large angular changes, it's ridiculous not to.
Same is true for ends of Hohmann transfers. Locking to prograde/retrograde is more efficient than a fixed vector.
It should be verified mathematically, but I'm confident that the same general principle is applicable for all short maneuvers (where burn time is a quarter orbit or less). There is a most efficient maneuver vector, but it isn't fixed to the orbiting body. It's a heading relative to prograde and it keeps that same relationship to prograde as the maneuver continues.
I don't pretend to understand the details for long burns. I excelled at differential equations in school, but the math for full revolution continuous power burns is currently beyond me.
On the throttle: The maneuver planner assumes that all engines enabled in the current stage (and only those engines) will be throttled to the current thrust limiter. When I want a small burn, I have to run around to the relevant engines and adjust the thrust limiters in order to draw out the burn to something I can manage. I'd prefer that you could adjust the throttle in the planner (and that it would be a temporary override of the actual limiter).
A Kerbonautic Maneuver Planner
An ideal maneuver planner needs to work from the desired end conditions (a particular orbit around a particular body) and calculate back towards the vehicle controls (vector, throttle, start time, end time). To get into a non-intersecting orbit will require two or more burns. There are additional trade-offs (thrust vs delta-v vs time, gravity slingshots, etc) which will manifest as additional player choices. Sources of error (staging, engine failure, rounding issues) could result in residuals and the need for occasional correction burns. The SOI simplification, which I accept as essential to KSP playability, means that we aren't trying to replicate a NASA/JPL maneuver planner. This solution will be specific to KSP.
I'd appreciate any pointers to ongoing relevant discussions in the replies. Very happy if this gets folded into an existing thread covering similar thoughts.