While it's hard to judge balance, especially when it's not clear what you're balancing against, these two house-rules both have a pretty effective point of comparison... in an older edition.

In v3.5 of Dungeons & Dragons, Wizards had the option to something like what you suggest. They could opt to prepare only part of their spells and keep the rest open, so they could be filled in later. Wizards were considered one of the most ridiculously powerful and flexible classes in the game.

What you are proposing is even more powerful, because the Wizard at least required at least 15 minutes of rest to prepare a new spell in an open slot.

In my games, I've seen this feature used by a few players. (Most opted to just prep everything at the start of the day.) Here's what happened:

• Gameplay slowed down. Whenever my players would run into a problem that they could conceivably solve half an hour later, the Wizard would go down his spellbook and look for possible solutions that they could prep. Wizards tend to have a lot of spells in there, so this would take a while. Lots of banter would ensue over which spell the Wizard might prep. Half the time, in the end no spell was prepped and a different solution was chosen altogether.

• Players would look to the Wizard even more than normal. A Wizard who preps everything at the start of the day is incredibly flexible. A Wizard who can prep anything once you know what you're up against is just unreasonably flexible.

• The Wizard would ultimately stop using the feature because it's a huge mental load. As the spellbook grows, the number of things a Wizard could do in a given situations rises very rapidly. It was doable for a few levels, but it quickly became too much.

In the end, most players chose not to use the ability because it was both too good and too cumbersome. It would take too much table-time away from having fun and made a lot of encounters too easy.

You are likely to experience the same things as level goes up; the caster's options will go up rapidly, which increases their cognitive load and makes them much more powerful. If you think picking what to do in combat is bad when you have 5 spells to pick from, imagine how bad it gets when you have your entire spell list to chose from.

One of the reasons 5e cut down on how many spells you can cast at a given moment is because combat flows a bit more smoothly that way. And, like I said in my comment, there is a reward in actually thinking ahead now that you will lose. It's not your fault (or problem) that your players aren't gathering information about what they will be facing.

(As for hounding players to select spells; I always rule that after a long rest you prep the same spells as the day before unless you tell me you want to make changes. Most players have a default loadout that they use most of the time anyway).

Doing so would massively increase the power of spells with bonus action control mechanics and create some other issues that utilize that feature.

Normally, the rules are very strict in that you only ever get one bonus action. If you were to convert that bonus action to an action, then there would be instances (Action Surge, Haste, etc.) that would then allow you to do this more than once.

The specific impact will be variable based on the specific bonus action under consideration, but changing it to a mechanic that would enable more than one could be very problematic.

I will review several interactions below that highlight the problem. The spellcasting actions are most problematic not because of being able to cast more than one spell (or not), but because many spells utilize the bonus action mechanic to control the spell. If you can use that mechanic twice, you are doubling the output of every spell that has that control mechanism.

Ready Action and Bonus Actions

Typically, you can not use a Bonus Action in a Ready Action. If you can convert your Bonus Action to Action, then it potentially works with this mechanic as well - giving further flexibility in how you use it and allowing for greater use/misuse of the secondary damage dealing that I cover later in this answer.

Bonus Action control mechanics and Bardic Inspiration* are two things that would benefit immensely from this

Bardic Inspiration

For instance, a Bard could now be able to hand out two Bardic Inspiration dice in one round. This doubles their current limit of one die.

Being able to hand out multiple instances of this limited resource could sway an encounter by boosting your party's effectiveness at twice the speed.

Spiritual Weapon/Heat Metal (or any other bonus action damage/attack mechanic)

Giving more than one Bonus Action here will allow multiple instances of damage delivery from these spells. If upcast, you could be looking at a max of 5d8+Modifier for spiritual weapon or 9d8 for Heat Metal. Any spell that allows a delivery of damage via bonus action would benefit and would become significantly overpowered as it doubles the damage output over the length of the spell (generally 10 rounds.) Especially if you compare that against other 9th level spells like Meteor Swarm. (Big Thanks to Axoren for this inclusion!)

To be clear, this is not casting the same spell twice, or overlapping effects. It is using the bonus action control aspect of these spells twice.

The damage output of spells that use bonus actions to damage would increase

In direct comparison, let's look at the damage outputs (assuming failed saves and successful attacks) for some of these to highlight the power of allowing two bonus actions to deliver damage from an ongoing spell/effect.

Meteor Storm as our Control example

Meteor Storm is one of the most powerful damage dealing spells available. I'm going to use the damage output from this as my measuring stick to compare against the effects of a dual bonus action mechanic.

Meteor Storm is an AOE, so it could effect as many creatures as you can fit within it, but each one would get a total of 40d6 damage. That's an average damage of 140 per creature in the range.

Spiritual Weapon

Spiritual Weapon is a single attack spell, but lasts for 1 minute with an average damage of 27.5 (assuming +5 modifier) being delivered twice for 55 damage per turn for a 550 total over duration. In order for Meteor Swarm to be outperform this, you'd need to get 3 targets which may be difficult to achieve.

Bigby's Hand

Bigby's Hand provides another OP option. In this case, the hand is much more versatile. It can be used via a bonus action to act as a controller (interposing itself between two creatures or shoving them) and as damage (Clenched Fist/grasping). Utilizing the single hand to have two potential targets alone is very powerful. If we look at the damage dealing for Clenched Fist, we have a delivery of 108 damage per turn or 1,080 over the duration. Meteor Storm would need to target 8 creatures in it's area in order to have a similar output.

Heat Metal

Heat Metal is only effective against someone wearing armor, but in this case it would deliver 81 fire damage per turn guaranteed. Meteor Swarm would have to target more than 10 to become equivalent.

Dual Bonus Actions are very powerful

While you may get 5+ targets in a Meteor Swarm attack, it's highly unlikely that they'd be arrayed in such a way and one of the most powerful spells in the book becomes dwarfed by many other options.

But the real kicker is you are now spending a minute dealing massive amounts of damage with only a single resource having been spent (especially for things that don't involve concentration where there is no risk of not completing the full minute.) The economy of doing that is overpowered.

Not only all of the above, but the one class ability where you could start to challenge the need for this is with the Potent Spellcasting. This class feature is a method for increasing the DPR of that specific class. If you do allow two bonus actions, thjen it completely negates the increase that Potent Spellcasting provides and greatly minimizes that level 8 class ability.

*^{Although Bardic Inspiration uses a limited resource, it shows that it removes limits on other features as well.}