I feel like those 3 bottom suggestions just raise the 'starting point' of when you can salvage. The mechanic was made so that the player has an easier time building their fleet.
Requiring a tug or salvage gantry could easily hurt a new player financially with their associated fuel costs. And requiring that you spend supplies and heavy machinery on recovering would do the same.
If I can ask, what is the end goal with these suggestions, because it just looks like it's making recovery hard for small fleets, and not really a problem for larger ones.
The answer to that question depends on when it's asked. I actually started this suggestion off just with logic in mind - The operation wouldn't be instantaneous - then, the gameplay scenarios it'd create - "Oh cool, I found an Onslaught I can recover! Now I just have to hope that a pirate fleet doesn't discover me before I bring it online and get out of here"; which might well result in a battle over the derelict.
To go off on a tangent, such a battle would be pretty interesting, it wouldn't have to be an absolute victory or defeat - Let's say the derelict is in the battle map. Your construction rig would be deployed by default, and you have to defend it and the derelict while the recovery operation is underway - this provides a tangible and relevant point of interest/objective within the battlespace. Once the rig has done its job, the derelict can then retreat from the battle under its own power, and you could withdraw the rest of the forces at will (Then forcing you into a fighting retreat with a mothballed ship to babysit, should the enemy force retain enough strength to pursue*). You could also choose to hold the rig in reserve and go for that absolute victory, or deploy it mid-way through a battle once you feel it's safe enough.
If it's a salvage fleet you're in conflict with, they might directly contest the derelict themselves, bringing a tug or a con-rig to steal it from you, making these civilian ships that would otherwise be irrelevant another emergent objective.
*This would also result in a soft limit of the types of derelicts you can get away with salvaging. If you've got the one rig and few supporting skills, and an otherwise shabby fleet, it's going to take longer to bring online, and your ability to protect it will be diminished, obviously scaling with the size/complexity of the derelict in question. Depending on the exact scenario, a player may be compelled to abandon the effort entirely**
Also note that I mentioned that ships with salvage gantries could serve the role of construction rigs, at an efficiency penalty of some sort - the game currently lets you start with one.
Now the only weak point of this is the question of how often those battles would happen; how often your fleet would be directly threatened while performing a recovery operation, out of the hundreds of ships you can be salvaging without seeing a single passing fleet, hostile or not.
This would go a ways to resolving this and, back to your question, what I feel is the core issue with the salvage system - Recoverable derelicts are far too common and don't demand the risk or sacrifice of anything significant(**), compared to the reward of what are essentially brand new ships you'd have to otherwise earn money, and/or reputation, and/or suspicion to gain.