While I understand the goal of restricting how many spells are available at once, it effectively makes Strength the stat that governs this which doesn't feel right to me.
I appreciate that there are options like porters which mitigate this, but at low levels it seems to punish magic users even more than most OSR systems and dissuades the use of niche spells like 'Ventriloquism' or 'Read Languages' when you can only carry 2 or 3 total.
Going from level 1 to level 6, unless strength is a primary, then an arcane magic user will have 5 more spells per day and 60 more spells to choose from but only 3 more slots. Meanwhile a melee fighter is unlikely to ever need more slots than they start with besides perhaps having a secondary weapon or acting as the party's pack mule.