I'll repost most of the mechanical differences:
• Alignment to Sanctification: Alignment no longer exists. Instead of choosing an Alignment, you may dictate any number of edicts and anathema that bind your character’s actions. Each Alignment was replaced with a corresponding Sanctification: Good Alignment to Holy Sanctification, Evil Alignment to Unholy Sanctification. Lawful and Chaotic alignments were removed, and not associated with any sanctifications. Each Sanctification has its own corresponding trait, and interacts with abilities with the Sanctified trait. Alignment damage was replaced with spirit damage, and alignment resistance was replaced with resistance to spirit damage and resistance to attacks with the corresponding Sanctification trait. See Sanctification Overhaul on page 41 for more information.
• No Spell Components: Spell components were removed. Spells with material or somatic components instead have the Manipulate trait. Spells with a verbal component instead have the Concentrate trait. Spell Component Pouches were removed from the game. Primal Symbols and Religious Symbols are no longer necessary to cast spells for Clerics and Druids, but may have other uses.
• No Spell Schools: Spell Schools no longer exist, and you need not keep track of a Legacy spell’s school. Wizards replaced Spell Schools with Arcane Schools. Arcane Schools contain spell curriculums which act as more freeform spell schools defined by a given theme, allowing overlap of one spell over multiple Spell Schools. Drop any Spell School traits (Abjuration, Conjuration, Divination, Enchantment, Evocation, Necromancy, Transmutation) from any spells when converting. The Illusion trait was an exception that survived into the Remaster.
• No Activation Components: Item activation components were simplified. The Command component was replaced with the Concentrate trait, and no longer has the Auditory trait. The Envision component was replaced with the Concentrate trait. The Interact component was replaced with the Manipulate trait.
• Positive/Negative to Vitality/Void: Positive effects are now Vitality effects, and Negative effects are now Void effects.
There is one additional thematic difference, GM side, which is Paizo lost a few OGL creatures moving into Remaster. And these are the few reasons you'd ever consider at least a Bestiary book over a Monster Core. But some creatures that are distinctly D&D, like metallic and chromatic dragons were replaced with legally distinct versions. Chromatics largely got very direct conversions, like say red dragon to cinder dragon. But metallics have less obvious conversions. But you're better off, and gonna have an easier time sticking to Remaster, these thematic elements are only worth gathering up if you are a huge D&D traditionalist. And even then, you can access them from Archives of Nethys.
For the most part, Legacy books can be ignored, one exception is that the GameMastery Guide from Legacy has a large number of optional rules that are not in the GM Core which might or might not be of interest: But you can find them on Archives of Nethys.These are:
- Gradual Ability Boosts: https://2e.aonprd.com/Rules.aspx?ID=1300
- Point Buy: https://2e.aonprd.com/Rules.aspx?ID=1301
- Alternative Scores: https://2e.aonprd.com/Rules.aspx?ID=1306
- Alignment Variants (This one is largely defunct unless you choose to bring alignment back): https://2e.aonprd.com/Rules.aspx?ID=1307
- Deep Backgrounds: https://2e.aonprd.com/Rules.aspx?ID=1318
- Dual-Class PCs (basically Gestalt rules): https://2e.aonprd.com/Rules.aspx?ID=1328
- Ancestry Paragon (bonus ancestry feats): https://2e.aonprd.com/Rules.aspx?ID=1336
- Simplified Ancestries: https://2e.aonprd.com/Rules.aspx?ID=1339
- Simplified Skill Feats: https://2e.aonprd.com/Rules.aspx?ID=1341
- High-Quality Equipment: https://2e.aonprd.com/Rules.aspx?ID=1367
- Skill Points: https://2e.aonprd.com/Rules.aspx?ID=1373
- Stamina (based on Starfinder 1E's Stamina system): https://2e.aonprd.com/Rules.aspx?ID=1378
