I would argue that there is no true rest apart from Jesus Christ and the rest being spoken of here is something that Joshua would have been incapable of giving to them.
Come unto me, all ye that labour and are heavy laden, and I will give you rest.
Matthew 11:28
Paul's teachings can be a little convoluted at times. Even Peter wrote this: "2 Peter 3:15-16 And account that the long-suffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction."
Peter is suggesting here that those who don't get it shouldn't change it to suit themselves or they may be in some trouble.
"Heb. 4:8 For if Jesus had given them rest, then would he not afterward have spoken of another day."
This can't be understood when taken out of context.
"Heb.3:1-19 Wherefore,---consider the Apostle and High Priest of our profession, Christ Jesus (he who we have professed our faith in); 2 Who was faithful to him that appointed him (our Holy Father sent him to accomplish his will in granting us salvation),---4 ---he that built all things is God.----6 But Christ as a son over his own house, whose house are we, if we hold fast the confidence and the rejoicing of the hope (our salvation) firm unto the end. 7 Wherefore as the Holy Ghost saith, To-day if ye will hear his voice, 8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness: 9 When your fathers tempted me, proved me, and saw my works forty years. 10 Wherefore I was grieved with that generation,--- 11 So I sware in my wrath, They shall not enter into my rest. 12 Take heed,---lest there be in any of you an evil heart of unbelief, in departing from the living God. 13 But exhort one another daily, while it is called To-day; lest any of you be hardened through the deceitfulness of sin. 14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;-----17 But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness? 18 And to whom sware he that they should not enter into his rest, but to them that believed not? 19 So we see that they could not enter in because of unbelief. Heb. 4:1 Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it. 2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it. 3 For we which have believed do enter into rest,----4 For he spake in a certain place of the seventh day--And God did rest the seventh day from all his works. 5 And--again, If they shall enter into my rest. 6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief: 7 Again---saying in David,---To-day if ye will hear his voice, harden not your hearts. 8 For if Jesus had given them rest, then would he not afterward have spoken of another day.------11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.----14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession (our faith)."
"another day" written in Heb. 4:8 refers to that time which is called the day when God was provoked to anger in the desert by those who did not believe (Heb. 3:8), and also of the seventh day of God's rest from all his works which he made and how many could not enter into his rest because of unbelief (Heb. 4:4-6), and as written by David in "Psalm 95:7 For he is our God; and we are the people of his pasture, and the sheep of his hand. To-day if ye will hear his voice (believe)." Those who do not believe can not enter in. Jesus is our salvation if we will believe. To-day is our day of salvation or not. For those who are in unbelief, it is not.
"another day" is referring to those examples of people provoking God to anger by their unbelief. God has given us Jesus as our salvation, but we must believe that he is.
Heb. 4:8 For if they had chosen salvation, then God would not afterward have been angered as at other times.
"Heb. 4:9 There remaineth therefore a rest to the people of God (but you must choose him)."
"Heb. 4:11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief (as at those other times)."