There's nothing wrong with using 5E D&D spells with A5E. We have done it for two long running A5E campaigns, and it has worked well in both of them. That said, 5E spells don't list the non-classical spell schools, so you'll need to look at what makes sense.
A long list of 5E spells are here...
Looking at the A5E spell Find Steed, it would be great if there were some additional non-fiendish options when using a 4th level spell slot. Serathiel’s Find Fiendish Steed has these options for the rare variant of the spell, when using a 4th level spell slot.
CR 3 Hell Hound
CR 3 Nightmare...
Sure, but in that case don't invest a feat until you are ready. This is one of the motivations to start characters at level 1, to give players, particularly new players, time to figure out how they want to play the character and where they want to focus.
I'd argue that for most classes, the simplest and best choice for feats is +2 in your primary ability score until you get it from 16 to 20. The first time a feat becomes competitive in many cases is at character level 12. Yes, you can have a specialized build that uses say Polearm Master, but I...
In my opinion, you switch it out when you get a better magic item. If you take Polearm Master, you're assuming that magical polearms will be available in your game world, which may or may not be the case depending on your DM. If you're in a story where most of the magic weapons found are swords...
When I've had a character with a familiar available, the familiar is often used to scout ahead. This is both outdoors and in dungeons, at the behest of the party. A lost familiar is cheap to replace (less so if you gear them up well to make them more effective), unlike a PC scout that goes down...
I would want a build something like Fighter 11/Rogue 9, Fighter 12/Rogue 8 if you want the extra feats, or Fighter 13/Rogue 7 for Studied Attacks at Fighter level 13. The big benefit of getting to Fighter 11+ is the extra attack at level 11 in Fighter. From an offensive standpoint, there's not...
Why not allow the players to set up a sufficiently hot fire with a Survival skill check, or Arcane skill check if using magic, and then have the character making a smithing tool proficiency roll? If the conditions are poor, impose disadvantage on the roll. But I would lead into the "Rule of...
Not quite what you were thinking of, but the Arcane Knight 4th level combat maneuver Flame Burst sets three squares within your reach as difficult terrain until the end of your next turn and "Creatures that start their turn within the flames take fire damage as though hit by your weapon, as does...
If you no longer have a feat, you no longer get the benefits of that feat. So, if you swap out the Lessons of the First Ones eldritch invocation that selected the origin Tough feat, you'd also lose any HP that had been previously gained.
If you have Pact of the Blade to use Charisma modifier instead for attack and damage, why not use Green-Flame Blade or Booming Blade, which scale better (+d8 at each higher tier) than True Strike (+d6 at each higher tier)? I'd be inclined to pick Agonizing Blast in Green-Flame Blade, which I...
I agree. There are always going to be levels where one character's powers are a bit better.
In that context, I prefer something like the 3E D&D Wizard's familiar that scales with the Wizard's level. A5E Pets & Sidekicks has a somewhat similar mechanism for scaling pets. This does make for a...
I agree, though this is an issue specific to D&D 2024 where it can be taken at level 1. In D&D 2014 and A5E, Warlocks don't get their Pact Boon until level 3. By character level 3, the Pact of the Chain improved familiars are at best on par with level 3 characters.