I seems to be a great combo to use 3 action Harm while you have an undead minion, to heal it while you harm your enemies. Unfortunately you need one of your actions to sustain Animate Dead.
Haste does not work, as the extra action cannot be used for sustain.
I found Effortless Concentration, unfortunately it is only available on level 16. Is there an earlier solution?
Clerics are the absolute masters of Heal/Harm, and, unsurprisingly, the ones that can provide you this answer. The level 14 cleric feat Fast Channel allows you to cast a three-action heal/harm with two actions. The action economy is pretty tight in Pathfinder 2, though. I wouldn't expect you to find anything that would do exactly what you're looking for all that much lower. That said, there are ways to get undead minions that do not require sustaining a spell.
Command Undead (Cleric feat, lvl 4) and Improved Command Undead (cleric feat, level 10) allows you to temporarily take control of an undead target. You can similarly get a harm-healed minion by turning a normal familiar or animal companion (available through archetype feats, if nothing else) into an undead with the lvl 5 Dhampir (ancestry) feat Undead Companion. You can create an undead from scratch with the level 2 ritual Create Undead, or (temporarily) with the consumable item Dust of Corpse Animation. All of those would allow you to have allied undead on the field while still permitting you your full action allotment, and are available well before level 14.
On level 2 you take Witch Dedication, on level 4 Basic Witchcraft for the Cackle feat. Cackle can sustain a spell as a free action for a Focus Point, so you have 3 actions to cast Harm.
Unfortunately it takes Int 14 (the not optimal part)
You cast Glyph of Warding on a container. When you open this container, a 3 action Harm is released.
Unfortunately, it needs to be of lower level than Glyph of Warding, so you cannot use your Harmful Font.
With the Fast Channel feat, as Ben Barden wrote.
It's not Animate Undead, but it's very thematically similar: the Undead Master archetype essentially gives you an Undead version of an animal companion. At Level 4, you can grab Mature Animal Companion, which gives your Undead minion one action per turn even if you don't command it. Very strong synergy with an Evil Cleric, but if you're doing PFS you'd have to take Beastmaster with the Undead Companion boon (thanks @András for pointing that out). In a home game, it can work very well and at early levels - and you can probably even get your GM to waive the alignment requirement.
