In the old games, waiting in the Adventurer's guild was good to replenish spell points.

It doesn't seems to work in the remake?

I'm pretty sure time only passed in the Adventurers' Guild in a few versions of the game, and thus spell points also only passed in those versions.

Every version I know of allowed you to regenerate spell points while immune to random monster attack by standing outside and bringing up your character screen. I don't think this works in the remaster, though.

I'm quite sure to have been attacked while looking at the character sheet :(

In the Remake ( BT1 ), are the only ways to recharge spell points is by paying Roscoe or waiting outside ?
In the original PC games, there were a few other options but I don't recall which of the three games had those other methods.

Waiting outside seems the only way of having SP restored.
In daylight.

If I switch between two buildings to get encounters in the city (and grind levels) it also seems to not work properly.

Even if you have your character sheet open, SP doens't recharge.

You could also go to Roscoe and have your spellpoints restored for 10 gold a spell point. Which isn't much past the first few levels.
And I think there's a chance of a random encounter every time you go into an empty house, so you could pick a spot with two houses that are oposite each other and just go back and forth to get some random encounters.

There's a couple other options. Specifically:
* In dungeons, there are spell point regen squares. I don't know of any locations in the first two games that qualify, but I believe the manual mentions them. Note that the SP regen is as slow as it is in daylight. (In BT3, there is a late-game spell that shows them on the map, and I remember finding one early in the second dungeon.)
* If you find a Mage Staff, equip it and you will regenerate SP as long as it is equipped; you also get 1 SP per combat round. (This was supposedly broken in the initial release of the remake, so you may need to update your game for this to work.)

In Bard's Tale 2 and 3, you can play the Rhyme of Duotime: It will make your SP regenerate faster. In 3 (at least the 8-bit versions, don't remember if this worked in the buggy DOS version), it will work even in dungeons or at night; that's not the case in 2.

In 3, Roscoe's is not available, but there are a few new options to make up for it:
* The old man at the review board will sometimes re-energize your spellcasters. This happens on the very first visit, as well as once after each world is cleared (when you turn in the artifacts and get the quest for the next world). This isn't something you can rely on, however.
* Harmonic Gems. Use one, and the character using it will recover all SP. There are also a couple late-game items that have this effect as well. (In the 8-bit versions (that is, the good versions), Harmonic Gems are *by far* the most common item in the entire game; you will likely find yoursellf with too many of them and need to drop or waste them as a result. There have been battles where I would get 2 or even 3 or them after a single battle. In other words, don't worry about wasting them; you'll find more, more than you can carry, in fact.)
* A few of the new items work like the Mage's Staff, but you might not need them if the game decides to flood you with Harmonic Gems.
* If you're desparate, I note that you don't get attacked if you don't move in BT3, but can still regenerate SP (plus there's a bard song that prevents outdoor encounters, and a couple towns with no encounters outside of buildings.)

By the way, one little bit I noticed in Bard's Tale 3:
* There are a few spots where you are expected (perhaps even required) to use a certain spell; I have noticed that they tend to also be SP regen squares. I think that's an interesting little bit of game design there.