To judge the strength of a planet, a better indicator always is to go off the person themselves and look at them. The reality of the actual physical situation. Then consider what this planet in question is doing secondly. If this planet is prominent and on/aspecting the angles and with positive aspects on it, this is a bigger indication of it being stronger than the sign it is in alone.
For example, if the Sun is in Leo in the 6th house, and isolated save for a Neptune aspect, along with maybe the Mars 10th house near the angle- being in a water sign with Saturn, this would point to a weaker solar plexus naturally. By being in a cadent house aspecting a malefic. This Mars (say if in Scorpio) could develop strongly though, and based on the dynamics of the chakras, Mars' feeds into the Sun's strength and this one could build it up.
But say also I know this person to be not a very self assertive person (typical early on in Mars/Saturn people) and they are very confused by their Sun/Neptune that they don't even consider assertion of themselves in the fullest light. Then I have to also consider this in how their Sacral/Mars would feed into the above chakra.
At the Sun's level in such a case regardless of the Leo position, Neptune's imposition on this sun throws off the natural "fixedness" of this planet in it's right sign. It is still much stronger here than in Pisces or something, but with a Sun/Neptune in Leo the person will be confused of their strong and willful side balancing with the Neptunian confusion, whereas a Sun in Pisces with this aspect would go for this more naturally and be more inclined to meditation, sensitivity etc. The upwards energy of Mars could flow into either confusion or creation here. So it swings in roundabouts.
Not only like the case of above, do I have to consider the sign position but also the aspects, the house placement, and also the blending of all these into my own level at understanding the dynamics of how they all play towards eachother in the souls workings and of reality like in how this person is social in the world, expresses themselves individually etc.
THEN there is also retrogrades+ stationary, fixed stars, special, critical and/or anaretic degrees(0' and 29'), over-presence of numerological consistencies, aspect type consistencies, whether a planet rules multiple houses (via it's signs, co-signs or even decanship) which are all incredibly acute and scrutinous details, but one of which could revolutionise this person's understanding of themselves if you were doing the chart for them, and met in these an awakening to this person's spiritual anatomy. There are many many things beyond even those also. You could also consider the planetary day and hour, the Moons phase and its position in the light, the Vedic classifications of certain parts of the signs etc. etc. Then how that blends in too, then the next stuff and blend it in...then the next stuff and blend... :lol:
You have to look at and consider how the chakras feed into eachother. Looking at the chart and how the patterns flow, and seeing the planets flowing as their respective chakras flow if that makes sense.
I wish I could write an example like above about all of them into one, one day. I need to progress my knowledge first and ability to convey this more simply too, and less rambling.
https://ancient-forums.com/viewtopic.php?f=8&t=42018
I wrote a post here about judging prominence of planets, which is essentially also prominence of chakras. I also made some replies on the Astrology board about the dynamics of the throat and heart of the implications given by the fact that they cross over in rulership, which might interest you.
To answer your question regarding former lifetimes and chakra work, chakras being strong can also occur from raw experiences through each life as well not just through work as often we led lives without. Chakras being strong in past lives I believe on the surface level is seen with the relationships of their respective planets, to the angles, and aspects with Jupiter + the other planets/chakras they gel with the most which would be benefic for them.
If the planet is in detriment or fall this does not always 100% make it weaker considering the other stuff I wrote in the example. About Neptune, along with Pluto it rules only the ethereal and outerly portions of what the pineal gland is dealing with and looking at. The Moon is the eye itself which is looking and it rules the actual physicality and whole of the Pineal gland hence it also ruling the Pineal 6th chakra. Memories for example, the main functions of your Mind and your everyday emotions are not Neptunian-esque, ethereal, unless you are meditating that day in a deep state. Same with Pluto, unless on that day you are battling mentally and fighting for your life or something else that is strange and Plutonian.
The Moon has primacy over the Mind and Femininity and not the insanity of the Outer planets. It is the juggler between both these higher forces and has to be built up to deal with their intensity. Empowering our Mind means empowering the Moon which is closest to us, not the distant Outer planets.
The Personal planets(all up to Saturn) have primacy in the workings of the soul, the 3 Outer planets are secondary in consideration.
For your last questions, Yes, and it is the planets that are to do with blockages like Saturn, whose aspects can block chakras. A Pluto, Neptune or Uranus retrograde over a personal planet could also show this as well as other things like stars and bad degrees. Tight aspects between aspects can otherwise indicate a connection between 2 chakras on the energetic and functional level of the soul, even Saturn ones for the base chakra. These connections can be "harsh" depending on other things, but a square aspect alone isn't harsh even though a lot of people say stuff like this often about squares.
To unblock the chakras you don't use planetary squares, those give where the chakras are at currently, more energy which can be very negative unless affirmed for other stuff or affirmed in of themselves to clean. But that is pretty pointless, almost like why would you clean your car with paint? Might as well just clean it with the white-gold energy, focusing it into the relevant chakras and using Aum Suryae and other cleaning mantras. That's what unblocks them.
Hail Azazel!