The big thing from Harper is the INT to Hit and Damage and it takes 11 APs to get that. EK tree does have INT to hit but not damage. Since Wizard uses INT for spells and Rogue skills use it too, it really makes sense to make INT your main ability. But beyond those 11 most people don't spend more. EK should be your main enhancement if you want to do melee primarily, as you do.
With respect to PLs, there is no absolute right or wrong way to do them and people have different strategies. I would recommend looking the PLs over and thinking about what PLs would be best for the type of builds you like to play. So if you want to continue to focus on melee then there are certain PLs that are better; for casters there are others; and so on. While you do probably get greater impact by taking 3 together, if you hate playing a life or just want to change it up, you should do so.
https://ddowiki.com/page/Past_Life_Feats
So a few general things about PLs. First in any given life 1-20 lvl, you can take EITHER a race or class PL. The first racial PL in any race is pretty unimpressive--a +1 bonus in a skill. But the second life is better; a +1 bonus in an ability. And the third racial PL is very good; 1 enhancement point in the racial tree. So if you have 15 racial pls in five different races, you would start out with 5 bonus racial points; that's why it would be easier later to try and get Throw Your Weight Around without diluting the enhancement points you might need in other trees. People used to default to doing racial lives first because of this and also because racial completionist gives you an automatic feat with +2 for all abilities and all skills. Class completionist used to require you to use one of your feats but now it too is automatic. And it is easier to get class completionist because it requires only 1 life in each of class (15 lives) where racial completionist requires 3 in every race (42 lives).
In terms of your current life, if you took Wizard as your PL you would get a passive bonus of +2 spell penetration. If you took 3 wizard PLs you would have +6 spell pen and that would really help you with your spells. You could also take a Wizard feat (not granted--you have to spend a feat to get it) that is good for casters; a +1 to your spell DCs, and again, +3 DC if you have 3 wizard lives.
If you went with Dwarf instead, you would a +1 to balance (bleh), but your second dwarf life is a +1 in Con (good for any character), and your third dwarf life would get you a bonus racial enhancement point. Although this is good, I would suggest doing the Wizard first because it is one of the best class lives for anyone who does spell casting.
https://ddowiki.com/page/Racial_Past_Life_Feats
Epic PLs are also really powerful but obviously you have to go from 20-30 and as I said above, it probably makes sense to defer doing that until you get more confident in your character building and get better gear. Also getting the bonus ability points (32 instead of 28 after first PL and 34 after second) will make it easier to get through epic. After you get a few class or epic PLs though it can be helpful to get some Epic PLs because the epic PL stances are really powerful.
https://ddowiki.com/page/Epic_Past_Life_Feats
There are four epic spheres and you can have one stance active from each of them. Some of them are good in general, some are good for melee, some for ranged, some for casters and some healers. If you picked four Epic PLs, one from each sphere, and got 3 PLs in each, then you could have those four active stances and you would also get 1 enhancement point (1 AP for every 12 epic PLs).
There are also Iconic PLs that you get by going from 15-30 and the great thing about them is that when you TR at 30 you get BOTH the Iconic PL for whatever your iconic is, PLUS you also get a class PL! So you get two PLs for 15 levels, which is really efficient. On the other hand, the Iconic PLs are probably the least good. You can have ONE Iconic stance active though and it isn't a bad idea to get three Iconic PLs in whatever Iconic gives you the best stance for your toon. For example, Deep Gnome stance is really good for casters because it is +1 illusion DC and +5 acid spell power or +3 illusion and +15 acid power for three Deep Gnome lives; that can really make your color spray and your acid well spells powerful. Aasimar Scourge gives you +2% doublestrike and 3 of those lives is +6%! That can be really good for melee.
So it does help to know what you like to play and focus on those PLs first. Or alternatively, there are PLs that are good for all characters--with bonuses in Fortification or healing amp for example.
Bottom line on PLs is you do want to TR at 20 to get the 32 point build in your next life and either Dwarf or Wizard could be helpful. It might be better to get 3 of the same first but it isn't necessary. Do what is most fun and don't let PLs control your choices too much, but hopefully you will now make those choices knowing what your options are and what the impacts are.
In terms of shield mastery vs necro focus, you need to think about how you want to play and build accordingly. While Wizards are pretty multifaceted and can do both EK melee as well as caster, it does make sense to focus more on one. If you take necro but you still don't have enough spell pen or DC to make your necro spells hit for full damage, it might be a waste. And if you take some melee feats but not enough to crit enough, then that is sort of weak too. Does that makes sense. So I don't think there is a right or wrong answer but I would recommend deciding what you want to focus on. You can mix in some other types of feats because Wizard is multi-dimensional, but there is a point of diminishing return. So if you want to use shield and you want to focus on melee, I would stick with shield mastery.