Repugs don't get a say. Don't you remember the 2016 Republican primaries?
To answer your question, yes, he needs to be barred.
I have no idea why you think this (unless Trump is barred by the Senate). Trump still holds a good approval rating by the majority of Republican voters. He certainly would have the highest support amongst a field of potential candidates. Unless Trump is so highly damaged by criminal court cases in the interim, AND all anti-Trump Republican voters get behind one candidate, there is every chance that Trump would end up as their candidate. As to whether he would win the election is another, more complicated matter. But he surely would be helped by some state legislatures in passing new laws to make this more likely within their states. We have already seen what Arizona has tried to do, and the Louisiana House has voted to censure Mike Cassidy for voting in favour of the trial being constitutional.
I take on board both of your arguments, but don't think it's anything like as clear-cut as you infer that Trump would likely with the Repug nomination.
He was a political unknown in 2016, and many Repugs were prepared to give him a go, especially when there was such fervent adoration from the crazies that form his base. That sort of enthusiasm can be infectious, and many mainstream Repugs voted for him as the best placed nationally to defeat Clinton (who was favourite to win the Dem nomination at that time). Come the 2020 election, and many Repugs that voted for him did so only because he was the Repug candidate.
If he stands again, his army of crazies will still be a-whopping and a-hollering like the deranged knobheads they are, but a hell of a lot of ordinary Repugs will swerve him. Add to that the weight of the Repug establishment who will know that a coalition of hard-right crazies and a diminishing band of mainstream Repugs won't be anything like enough to win the Presidency, and there likely
would be some behind the scenes jiggery-pokery to try to stop him winning the nomination.
There's several different permutations that could happen, and I think privately most Repug politicians will want him barred - but have to keep up the pretence that they still support him.
I just want to see the Repug vote split, whilst hoping the Democrats are able to reverse the regressive Trump/Repug policies on taxation & spending. My greatest wish is the Dems reversing the amendments to inheritance taxes.